Five things you need to know about CE Marked Wire and Cable

Wire is simple right? Maybe not! Here are the top five things to know when selecting wire and cable products for use in designs that will be CE Marked:

  1. Wire and cable products sold in the EU must be CE Marked under the Low Voltage Directive, and MAY bear BOTH the CE Mark and the HAR mark. The HAR mark may only be applied by manufacturers that have met the requirements for the use of the HAR mark. More information on the HAR mark. 

    Picture of the HAR Mark.
    The HAR Mark
  2. The HD 21.X and HD 22.X Harmonization Documents previously used for determining compliance and applying the CE Mark are being replaced by the EN 50525.X family of standards starting on 2014-01-17. See the list.
  3. Wire and Cable products with Declarations of Conformity that refer to older versions of the Low Voltage Directive, or that refer to HD documents that have been superseded are NO LONGER COMPLIANT.
  4. Wire and Cable products used in “large-scale” machine tools and fixed installations do not need to meet WEEE requirements.
  5. Designers are not required to use CE Marked wire and cable products in CE Marked Products.

Need to know more? Check out this article!

CE Marking Wire and Cable – Necessity or Luxury?

When I set out to investigate the need for CE Marks and <HAR> marks on wire and cable products, I would not have guessed that it would turn out to be as much of an odyssey as it did. For most products, determining the need for a CE Mark is relatively straightforward, but not for wire and cable products! As equipment designers, engineers and technologists, we rarely think much about wire and cable. We’re mostly concerned with the insulation colours, number of conductors, the gauge, and the voltage rating. Sometimes we’re also concerned about the temperature rating, the flexibility, or perhaps the shielding. The regulatory approvals carried by the wire are often assumed, or not considered at all. This common product can bring a world of headaches if the requirements are not fully considered.

Regulatory Requirements

North America

In North America, the three main regulatory organizations for electrical component safety certifications are UL, CSA, and NOM. All three publish standards applicable to wire and cable, and the markings and common wire styles, like TEW, AWM, MTW, and SOW, are driven by these standards.

Europe – HAR Marking

What about Europe? The EU has a separate system for identifying wire and cable, identified by the HAR Mark. Learn more about this mark.

Picture of the HAR Mark.
The <HAR> Mark

The HAR mark, which stands for “Harmonised”, has been based on the application of Harmonised Documents (HD) published by CENELEC, including the HD 21.X and HD 22.X families, which was replaced by the EN 50525 series of standards effective 2014-01-17. But what is the basis for marking, and is there a legal requirement for manufacturers to use marked wire? The HAR Mark is one of the earliest common marks in the EU, originating from an agreement signed in 1974. Manufacturers who wish to use the HAR Mark are required to meet stringent quality control requirements before being granted the right to use the HAR mark. Wire and cable products bearing the HAR mark are accepted by all of the signatory states to the HAR agreement. (Need to know more? Have a look at the EEPCA web site.) The HAR mark is not legally required, but using products bearing the HAR Mark may make a manufacturer’s life a bit easier when dealing with authorities.

Europe – CE Marking

What about the CE Mark for wire and cable?  To answer that question, we need to look at the CE Marking requirements in more detail. In general, CE Marking Directives are aimed at products, not at components, although there are some exceptions. Wire and cable products are one of those exceptions that stand out. On its own, wire or cable has no defined use or application, in that it must be built into something to be useful. The compliance of the final product containing the wire products is determined based on testing related to the finished product, and the compliance of the wire used in the product is based on the specific application and the wire product’s performance in that product. So why are wire and cable products CE Marked on their own?

Determining the Right Directives

Most directives require that products within the scope have some defined function, like the Machinery Directive’s definition of a machine:

“…an assembly, fitted with or intended to be fitted with a drive system other than directly applied human or animal effort, consisting of linked parts or components, at least one of which moves, and which are joined together for a specific application…”, [1]

or the EMC Directive definition of “apparatus”:

“…‘apparatus’ means any finished appliance or combination thereof made commercially available as a single functional unit, intended for the end user and liable to generate electromagnetic disturbance, or the performance of which is liable to be affected by such disturbance…” [2]

Clearly, these definitions don’t include components. So what directives do apply to wire products? The first directive that comes to mind is the Low Voltage Directive. If we take a look at the definitions in the Directive [3] we find:

Article 1

For the purposes of this Directive, ‘electrical equipment’ means any equipment designed for use with a voltage rating of between 50 and 1 000 V for alternating current and between 75 and 1 500 V for direct current, other than the equipment and phenomena listed in Annex II.

Once again, we have a pretty specific definition for the target of the Directive: “electrical equipment”. Or do we? What, exactly, is “electrical equipment”? The Directive doesn’t define this term, but it does give us a list of exclusions in Annex II [3]:

Annex II

  • Equipment and Phenomena outside the Scope of this Directive
  • Electrical equipment for use in an explosive atmosphere
  • Electrical equipment for radiology and medical purposes
  • Electrical parts for goods and passenger lifts
  • Electricity meters
  • Plugs and socket outlets for domestic use
  • Electric fence controllers
  • Radio-electrical interference
  • Specialised electrical equipment, for use on ships, aircraft or railways, which complies with the safety provisions drawn up by international bodies in which the Member States participate.

At this point, it doesn’t look like wire products are included in the directive. No further definition of “electrical equipment” is given, and wire and cable are not specifically excluded in Annex II. Where do we go from here to better understand the definition of “electrical equipment”?

The IEC publishes the International Electrotechnical Vocabulary (IEV), IEC 60050 [4], defining hundreds of terms related to electro-technical topics. This is the next logical step in trying to understand what is covered. Definitions in the IEV are numbered as a means to catalog the terms, and I’ve provided the definition numbers for reference. Unfortunately, the IEV does not contain a definition for “electrical equipment”, but it does define “equipment” [3, 151-11-25]:

equipment – single apparatus or set of devices or apparatuses, or the set of main devices of an installation, or all devices necessary to perform a specific task

Note – Examples of equipment are a power transformer, the equipment of a substation, measuring equipment.

The definition uses the term “apparatus”, which continues the lack of clarity. Is wire apparatus? Looking up the definition for “Apparatus” 151-11-22, the IEV gives us:

apparatus – device or assembly of devices which can be used as an independent unit for specific functions

Note – In English, the term “apparatus” sometimes implies use by skilled persons for professional purposes.

Wire clearly doesn’t meet the definition for apparatus, since it couldn’t be considered and “independent unit for a specific function”, so is wire a device? Now we have one more term to try to understand. The definition for “device” is found in the IEV at 151-11-20:

device – material element or assembly of such elements intended to perform a required function

Note – A device may form part of a larger device.

Now we’re getting somewhere. Wire could definitely be considered to be a “material element”, but we’re stuck again at the need to “perform a required function”. One more term might apply. Let’s look at “components”. The definition for a “component” is found at 151-11-21:

component – constituent part of a device which cannot be physically divided into smaller parts without losing its particular function

Now we’ve got it! Wire is clearly a component, and this clearly makes sense when you consider the use we make of wire and cable products. But how does this relate back to the legal definition of “electrical equipment”? Since the IEV is not called out by the Directive, we can’t lean on this definition alone to decide the applicability of the CE Mark to these products.

Low Voltage Directive Requirements

The EU Commission publishes a Guide for most of the Directives, and the Low Voltage Directive is no different. There is little direct reference to wire and cable products, however, [5, para. 8] does mention it in broad terms, “…the Directive covers consumer and capital goods designed to operate within those voltage limits, including in particular, …electrical wiring, appliance couplers and cord sets…” [5, Annex II] provides a pictorial list of products, illustrating the cord set requirement. Within the voltage limits set by the scope of the LVD, the requirement for cord sets and other “safety critical” sub-assemblies that include wire or cable makes sense. A completed cord set with an IEC 320 connector on one end and a country specific plug, like a a CEE plug cap, is a complete product with a defined end-use, and so fits the scope. This seems to answer the original question: “Do wire & cable products, on their own, require a CE Mark?”, at least under the LVD. The next question must be: “Are there any other CE Marking Directives that might apply?”

RoHS and WEEE Directives

We can exclude the EMC Directive, since the definition of apparatus in that directive is quite clear. What about RoHS [6], and WEEE [7]? Let’s look at RoHS and WEEE together, since these two Directives are linked in application. The 2011 RoHS directive [8] includes some definitions of what electrical and electronic equipment is, and includes two key definitions for machine builders:

Article 3 

Definitions

For the purposes of this Directive, the following definitions shall apply:

  1. electrical and electronic equipment’ or ‘EEE’ means equipment which is dependent on electric currents or electromagnetic fields in order to work properly and equipment for the generation, transfer and measurement of such currents and fields and designed for use with a voltage rating not exceeding 1 000 volts for alternating current and 1 500 volts for direct current; 
  2. for the purposes of point 1, ‘dependent ‘ means, with regard to EEE, needing electric currents or electromagnetic fields to fulfil at least one intended function; 
  3. ‘large-scale stationary industrial tools’ means a large-scale assembly of machines, equipment, and/or components, functioning together for a specific application, permanently installed and de-installed by professionals at a given place, and used and maintained by professionals in an industrial manufacturing facility or research and development facility; 
  4. ‘large-scale fixed installation’ means a large-scale combination of several types of apparatus and, where applicable, other devices, which are assembled and installed by professionals, intended to be used permanently in a pre-defined and dedicated location, and de-installed by professionals; 
  5. ‘cables’ means all cables with a rated voltage of less than 250 volts that serve as a connection or an extension to connect EEE to the electrical outlet or to connect two or more EEE to each other; [7]

The term “large-scale” is never defined in the directive. So what is “Large-scale” when it comes to machine tools? An explanation of the term is given in two places, [9] and [11]. The overall descriptions get a bit involved, but essentially it comes down to products that weigh 3 tons or more, or are at least 2.5 m x 2.5 m. Anything smaller than this is not considered “large-scale” and is therefore within the scope of the WEEE Directive. Some examples of “large-scale stationary industrial tools” include [9]:

  • Machines for the industrial production and processing of materials and goods, such as
    • CNC lathes;
    • Bridge-type milling and drilling machines;
    • Metal forming presses;
    • Newspaper printing presses;
  • Machines for the testing of work pieces, such as
    • Electron beam, laser, bright light, and deep ultra violet defect detection systems;
    • Automated integrated circuit board and printed wiring board testers;
  • Cranes;
  • Other machinery of similar size, complexity and weight.

What then, is a “large-scale fixed installation”? [9] can help us out here too. Some examples are given in the FAQ:

  • Production and processing lines, including robots and machine tools (industrial, food, print media etc.);
  • Passenger lifts;
  • Conveyor transport systems;
  • Automated storage systems;
  • Electrical distribution systems such as generators;
  • Railway signalling infrastructure;
  • Fixed installed cooling, air conditioning, and refrigerating systems or heating systems designed exclusively for non-residential use.

So, machine tools that weigh less than 3 tons, or are smaller than 2.5 x 2.5 m, are included in the scope of the RoHS directives, but machines larger that this, or systems that fit the descriptions of Large Scale Fixed Installations are out. What about WEEE? The WEEE Directive gives us some similar definitions in Article 3:

For the purposes of this Directive, the following definitions shall apply:

  1. ‘large-scale stationary industrial tools’ means a large size assembly of machines, equipment, and/or components, functioning together for a specific application, permanently installed and de-installed by professionals at a given place, and used and maintained by professionals in an industrial manufacturing facility or research and development facility;
  2. ‘large-scale fixed installation’ means a large-size combination of several types of apparatus and, where applicable, other devices, which: 
  3. are assembled, installed and de-installed by professionals; 
  4. are intended to be used permanently as part of a building or a structure at a pre-defined and dedicated location; and 
  5. can only be replaced by the same specifically designed equipment; 

WEEE also provides another list of products to consider in [8, Annexes I & II]. From the point of view of machine builders we need only look at Annex II, 6., which lists exclusions:

6. ELECTRICAL AND ELECTRONIC TOOLS (WITH THE EXCEPTION OF LARGE-SCALE STATIONARY INDUSTRIAL TOOLS)

  • Drills 
  • Saws 
  • Sewing machines 
  • Equipment for turning, milling, sanding, grinding, sawing, cutting, shearing, drilling, making holes, punching, folding, bending or similar processing of wood, metal and other materials 
  • Tools for riveting, nailing or screwing or removing rivets, nails, screws or similar uses 
  • Tools for welding, soldering or similar use 
  • Equipment for spraying, spreading, dispersing or other treatment of liquid or gaseous substances by other means 
  • Tools for mowing or other gardening activities 

If we take the interpretation of “large-scale” as [11], then it becomes clear that WEEE does not include most heavy machinery. Smaller equipment, i.e. not “large scale”, would be included. Seems clear enough, but how does this relate back to wire and cable?

In reading [9, Q5.2], we find that “Internal wires are not cables. Internal wiring in any EEE that is within the scope of RoHS 2 must simply meet the material restrictions like all other parts of the EEE; there is no individual CE marking and DoC requirement. If an EEE is subject to a transition period or a scope exclusion, the same applies to the internal wiring. The same principle applies to permanently attached cables, e.g. most lamp cables.” [9, Q5.3] continues this line of reasoning in relation to external cables, adding, “External cables that form part of another EEE because they are sold together or marketed/shipped for use with an EEE, e.g. power cords, must meet the material restrictions but do not need an individual CE marking and Declaration of Conformity if they are covered by the DoC for the EEE and the EEE is CE marked.” The comment regarding the applicability of the CE mark applies only to the RoHS Directive requirements.

Reading the definitions is never enough. The exclusions to the RoHS Directive [11, Art. 2] include some important points:

4. This Directive does not apply to:

c) equipment which is specifically designed, and is to be installed, as part of another type of equipment that is excluded or does not fall within the scope of this Directive, which can fulfil its function only if it is part of that equipment, and which can be replaced only by the same specifically designed equipment;
d) large-scale stationary industrial tools;
e) large-scale fixed installations;
j) equipment specifically designed solely for the purposes of research and development only made available on a business-to-business basis.

So machinery that is not either a large-scale stationary machine tool nor a large scale fixed installation is within the scope of the RoHS and WEEE Directives.

Summing Up

It looks like we have the full picture now, so let’s recap. Wire and cable products:

  • are included in the LVD, despite their usual classification as components, and therefore require CE Marking under this directive
  • are excluded from RoHS and WEEE when in component form,
  • are included in RoHS and WEEE when used in small-scale machinery (i.e., not large-scale stationary industrial tools or a large-scale fixed installation), consumer products, and medical devices that are not in-vitro or active implantable devices

So why are these products CE marked when in component form? The most obvious answer seems to be that some wire and cable products have been explicitly identified in the Commission Guidance on the Directive [5, Annex II]. Further, these products must always be incorporated into some other product, many of which are included in the scopes of LVD, RoHS and WEEE. In the case of the LVD, wire and cable products have a direct impact on the safety performance of many safety-critical assembles, like cord sets, so performance of the wire and cable product is essential to the safety of the end product.  It’s worth noting here that “cables” are included in the examples [5, Annex II], but “wire”, e.g., an individual insulated conductor, is not mentioned. This implies that wire does not need to be CE Marked as a component.

Is there a mandatory requirement for the use of CE Marked or marked wire and cable products? No. No more so that there is for any other component that may be selected for use in a CE Marked product. However, it is always recommended to use CE Marked components whenever they are available, as this reduces the likelihood of problems related to these products causing issues with the compliance of the final product.

Acknowledgements

I’d like to acknowledge the contributions of the following people to this article, and offer my thanks for their assistance. Some of those listed are members of the IEEE Product Safety Engineering Society, as well as members of the EMC-PSTC list:

Mr. Jon Cotman, Mr. Ted Eckert, Mr. John Gavilanes, Mr. Richard Robinson, Mr. Joshua Wiseman, Mr. John Woodgate.

References

[1] DIRECTIVE 2006/42/EC OF THE EUROPEAN PARLIAMENT AND OF THE COUNCIL of 17 May 2006 on machinery, and amending Directive 95/16/EC. Brussels: European Commission. 2006.

[2] DIRECTIVE 2004/108/EC OF THE EUROPEAN PARLIAMENT AND OF THE COUNCIL of 15 December 2004 on the approximation of the laws of the Member States relating to electromagnetic compatibility and repealing Directive 89/336/EEC. Brussels: European Commission. 2004.

[3] DIRECTIVE 2006/95/EC OF THE EUROPEAN PARLIAMENT AND OF THE COUNCIL of 12 December 2006 on the harmonisation of the laws of Member States relating to electrical equipment designed for use within certain voltage limits. Brussels: European Commission. 2006.

[4] International Electrotechnical Commission (IEC). “Electropedia: The World’s Online Electrotechnical Vocabulary,”  electropedia.org. [Online]. Available: http://www.electropedia.org [Accessed: 2013-12-19].

[5] L. Montoya, Ed. Guidelines on the Application of Directive 2006/95/EC (Electrical Equipment Designed for Use Within Certain Voltage Limits). August 2007 (Last Modified: January 2012). Available: http://ec.europa.eu/DocsRoom/documents/8716/attachments/1/translations/en/renditions/native. [Accessed: 2015-08-24].

[6] DIRECTIVE 2002/95/EC OF THE EUROPEAN PARLIAMENT AND OF THE COUNCIL of 27 January 2003 on the restriction of the use of certain hazardous substances in electrical and electronic equipment. Brussels: European Commission. 2002.

[7] DIRECTIVE 2012/19/EU OF THE EUROPEAN PARLIAMENT AND OF THE COUNCIL of 4 July 2012 on waste electrical and electronic equipment (WEEE). Brussels: European Commission. 2012.

[8] DIRECTIVE 2011/65/EU OF THE EUROPEAN PARLIAMENT AND OF THE COUNCIL of 8 June 2011 on the restriction of the use of certain hazardous substances in electrical and electronic equipment. Brussels: European Commission. 2011.

[9] RoHS 2 FAQ. European Commission, Directorate-General Environment. 2012. Available: http://ec.europa.eu/environment/waste/rohs_eee/pdf/faq.pdf. Accessed 2013-12-12.

[10] DIRECTIVE 2011/65/EU OF THE EUROPEAN PARLIAMENT AND OF THE COUNCIL of 8 June 2011 on the restriction of the use of certain hazardous substances in electrical and electronic equipment, 2011/65/EU. European Commission, Brussels. 2011.

[11] DRAFT Frequently Asked Questions on Directive 2012/19/EU on Waste Electrical and Electronic Equipment (‘new WEEE Directive’). European Commission, Directorate-General Environment. Unpublished.

IMAGES: Selection of wire and cable products, unknown source. HAR mark courtesy Ören Kablo.

Interlocking Devices: The Good, The Bad and the Ugly

This entry is part 1 of 2 in the series Guards and Guarding

Note: A shorter version of this article was published in the May-2012 edition of  Manufacturing Automation Magazine.

When designing safeguarding systems for machines, one of the basic building blocks is the movable guard. Movable guards can be doors, panels, gates or other physical barriers that can be opened without using tools. Every one of these guards needs to be interlocked with the machine control system so that the hazards covered by the guards will be effectively controlled when the guard is opened.

There are a number of important aspects to the design of movable guards. This article will focus on the selection of interlocking devices that are used with movable guards.

The Hierarchy of Controls

The Hierarchy of Controls as an inverted pyrimid.
Figure 1 – The Hierarchy of Controls

This article assumes that a risk assessment has been done as part of the design process. If you haven’t done a risk assessment first, start there, and then come back to this point in the process. You can find more  information on risk assessment methods in this post from 31-Jan-11. ISO 12100 [1] can also be used for guidance in this area.

The hierarchy of controls describes levels of controls that a machine designer can use to control the assessed risks. The hierarchy is defined in [1]. Designers are required to apply every level of the hierarchy in order, starting at the top. Each level is applied until the available measures are exhausted, or cannot be applied without destroying the purpose of the machine, allowing the designer to move to the next lower level.

Engineering controls are subdivided into a number of different sub-groups. Only movable guards are required to have interlocks. There are a number of similar types of guards that can be mistaken for movable guards, so let’s take a minute to look at a few important definitions.

Table 1 – Definitions

International [1] Canadian [2] USA [10]
3.27 guard physical barrier, designed as part of the machine to provide protection.NOTE 1 A guard may act either alone, in which case it is only effective when “closed” (for a movable guard) or “securely held in place” (for a fixed guard), or  in conjunction with an interlocking device with or without guard locking, in which case protection is ensured whatever the position of the guard.NOTE 2Depending on its construction, a guard may be described as, for example, casing, shield, cover, screen, door, enclosing guard.NOTE 3 The terms for types of guards are defined in 3.27.1 to 3.27.6. See also 6.3.3.2 and ISO 14120 for types of guards and their requirements. Guard — a part of machinery specifically used to provide protection by means of a physical barrier. Depending on its construction, a guard may be called a casing, screen, door, enclosing guard, etc. 3.22 guard: A barrier that prevents exposure to an identified hazard.E3.22 Sometimes referred to as barrier guard.”
3.27.4 interlocking guard guard associated with an interlocking device so that, together with the control system of the machine, the following functions are performed:

  • the hazardous machine functions “covered” by the guard cannot operate until the guard is closed,
  • if the guard is opened while hazardous machine functions are operating, a stop command is given, and
  • when the guard is closed, the hazardous machine functions “covered” by the guard can operate (the closure of the guard does not by itself start the hazardous machine functions)

NOTE ISO 14119 gives detailed provisions.

Interlocked barrier guard — a fixed or movable guard attached and interlocked in such a manner that the machine tool will not cycle or will not continue to cycle unless the guard itself or its hinged or movable section encloses the hazardous area. 3.32 interlocked barrier guard: A barrier, or section of a barrier, interfaced with the machine control system in such a manner as to prevent inadvertent access to the hazard.
3.27.2 movable guard
guard which can be opened without the use of tools
Movable guard — a guard generally connected by mechanical means (e.g., hinges or slides) to the machine frame or an adjacent fixed element and that can be opened without the use of tools. The opening and closing of this type of guard may be powered. 3.37 movable barrier device: A safeguarding device arranged to enclose the hazard area before machine motion can be initiated.E3.37 There are two types of movable barrier devices:

  • Type A, which encloses the hazard area during the complete machine cycle;
  • Type B, which encloses the hazard area during the hazardous portion of the machine cycle.
3.28.1 interlocking device (interlock)mechanical, electrical or other type of device, the purpose of which is to prevent the operation of hazardous machine functions under specified conditions (generally as long as a guard is not closed) Interlocking device (interlock) — a mechanical, electrical, or other type of device, the purpose of which is to prevent the operation of machine elements under specified conditions (usually when the guard is not closed). No definition
3.27.5 interlocking guard with guard locking guard associated with an interlocking device and a guard locking device so that, together with the control system of the machine, the following functions are performed:

  • the hazardous machine functions “covered” by the guard cannot operate until the guard is closed and locked,
  • the guard remains closed and locked until the risk due to the hazardous machine functions “covered” by the guard has disappeared, and
  • when the guard is closed and locked, the hazardous machine functions “covered” by the guard can operate (the closure and locking of the guard do not by themselves start the hazardous machine functions)

NOTE ISO 14119 gives detailed provisions.

Guard locking device — a device that is designed to hold the guard closed and locked until the hazard has ceased. No definition

As you can see from the definitions, movable guards can be opened without the use of tools, and are generally fixed to the machine along one edge. Movable guards are always associated with an interlocking device. Guard selection is covered very well in ISO 14120 [11]. This standard contains a flowchart that is invaluable for selecting the appropriate style of guard for a given application.

5% Discount on ISO and IEC Standards with code: CC2012

Though much emphasis is placed on the correct selection of these interlocking devices, they represent a very small portion of the hierarchy. It is their widespread use that makes them so important when it comes to safety system design.

Electrical vs. Mechanical Interlocks

Mechanical Interlocking
Figure 2 – Mechanical Interlocking

Most modern machines use electrical interlocks because the machine is fitted with an electrical control system, but it is entirely possible to interlock the power to the prime movers using mechanical means. This doesn’t affect the portion of the hierarchy involved, but it may affect the control reliability analysis that you need to do.

Mechanical Interlocks

Figure 2, from ISO 14119 [7, Fig. H.1, H.2 ], shows one example of a mechanical interlock.  In this case, when cam 2 is rotated into the position shown in a), the guard cannot be opened. Once the hazardous condition behind the guard is effectively controlled, cam 2 rotates to the position in b), and the guard can be opened.

Arrangements that use the open guard to physically block operation of the controls can also be used in this way. See Figure 3 [7, Fig. C.1, C.2].

Mechanical Interlocking using control devices
Figure 3 – Mechanical Interlocking using machine control devices

Fluid Power Interlocks

Figure 4, from [7, Fig. K.2], shows an example of two fluid-power valves used in complementary mode on a single sliding gate.

Hydraulic interlock from ISO 14119
Figure 4 – Example of a fluid power interlock

In this example, fluid can flow from the pressure supply (the circle with the dot in it at the bottom of the diagram) through the two valves to the prime-mover, which could be a cylinder, or a motor or some other device when the guard is closed (position ‘a’). There could be an additional control valve following the interlock that would provide the normal control mode for the device.

When the guard is opened (position ‘b’), the two valve spools shift to the second position, the lower valve blocks the pressure supply, and the upper valve vents the pressure in the circuit, helping to prevent unexpected motion from trapped energy.

If the spring in the upper valve fails, the lower spool will be driven by the gate into a position that will still block the pressure supply and vent the trapped energy in the circuit.

5% Discount on ISO and IEC Standards with code: CC2012

Electrical Interlocks

By far the majority of interlocks used on machinery are electrical. Electrical interlocks offer ease of installation, flexibility in selection of interlocking devices, and complexity from simple to extremely complex. The architectural categories cover any technology, whether it is mechanical, fluidic, or electrical, so let’s have a look at architectures first.

Architecture Categories

Comparing ANSI, CSA, and ISO Control Reliability Categories
Figure 5 – Control Reliability Categories

In Canada, CSA Z432 [2] and CSA Z434 [3] provide four categories of control reliability: simple, single channel, single-channel monitored and control reliable. In the U.S., the categories are very similar, with some differences in the definition for control reliable (see RIA R15.06, 1999). In the EU, there are five levels of control reliability, defined as Performance Levels (PL) given in ISO 13849-1 [4]: PL a, b, c, d and e. Underpinning these levels are five architectural categories: B, 1, 2, 3 and 4. Figure 5 shows how these architectures line up.

To add to the confusion, IEC 62061 [5] is another international control reliability standard that could be used. This standard defines reliability in terms of Safety Integrity Levels (SILs). These SILs do not line up exactly with the PLs in [4], but they are similar. [5] is based on IEC 61508 [6], a well-respected control reliability standard used in the process industries. [5] is not well suited to applications involving hydraulic or pneumatic elements.

The orange arrow in Figure 5 highlights the fact that the definition in the CSA standards results in a more reliable system than the ANSI/RIA definition because the CSA definition requires TWO (2) separate physical switches on the guard to meet the requirement, while the ANSI/RIA definition only requires redundant circuits, but makes no requirement for redundant devices. Note that the arrow representing the ANSI/RIA Control reliability category falls below the ISO Category 3 arrow due to this same detail in the definition.

Note that Figure 5 does not address the question of PL’s or SIL’s and how they relate to each other. That is a topic for another article!

The North American architectures deal primarily with electrical or fluid-power controls, while the EU system can accommodate electrical, fluid-power and mechanical systems.

From the single-channel-monitored or Category 2 level up, the systems are required to have testing built-in, enabling the detection of failures in the system. The level of fault tolerance increases as the category increases.

Interlocking devices

Interlocking devices are the components that are used to create the interlock between the safeguarding device and the machine’s power and control systems. Interlocking systems can be purely mechanical, purely electrical or a combination of these.

Roller cam switch used as part of a complementary interlock
Photo 1 – Roller Cam Switch

Most machinery has an electrical/electronic control system, and these systems are the most common way that machine hazards are controlled. Switches and sensors connected to these systems are the most common types of interlocking devices.

Interlocking devices can be something as simple as a micro-switch or a reed switch, or as complex as a non-contact sensor with an electromagnetic locking device.

Images of interlocking devices used in this article are representative of some of the types and manufacturers available, but should not be taken as an endorsement of any particular make or type of device. There are lots of manufacturers and unique models that can fit any given application, and most manufacturers have similar devices available.

Photo 1 shows a safety-rated, direct-drive roller cam switch used as half of a complementary switch arrangement on a gate interlock. The integrator failed to cover the switches to prevent intentional defeat in this application.

Micro-Switch used for interlocking
Photo 2 – Micro-Switch used for interlocking

Photo 2 shows a ‘microswitch’ used for interlocking a machine cover panel that is normally held in place with fasteners, and so is a ‘fixed guard’ as long as the fasteners require a tool to remove. Fixed guards do not require interlocks under most circumstances. Some product family standards do require interlocks on fixed guards due to the nature of the hazards involved.

Microswitches are not safety-rated and are not recommended for use in this application. They are easily defeated and tend to fail to danger in my experience.

Requirements for interlocking devices are published in a number of standards, but the key ones for industrial machinery are ISO 14119 [7], [2], and ANSI B11.0 [8]. These standards define the electrical and mechanical requirements, and in some cases the testing requirements, that devices intended for safety applications must meet before they can be classified as safety components.
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Typical plastic-bodied interlocking device
Photo 3 – Schmersal AZ15 plastic interlock switch

These devices are also integral to the reliability of the control systems into which they are integrated. Interlock devices, on their own, cannot meet a reliability rating above ISO 13849-1 Category 1, or CSA Z432-04 Single Channel. To understand this, consider that the definitions for Category 2, 3 and 4 all require the ability for the system to monitor and detect failures, and in Categories 3 & 4, to prevent the loss of the safety function. Similar requirements exist in CSA and ANSI’s “single-channel-monitored,” and “control-reliable” categories. Unless the interlock device has a monitoring system integrated into the device, these categories cannot be achieved.

Guard Locking

Interlocking devices are often used in conjunction with  guard locking. There are a few reasons why a designer might want to lock a guard closed, but the most common one is a lack of safety distance. In some cases the guard may be locked closed to protect the process rather than the operator, or for other reasons.

Interlock Device with Guard Locking
Photo 4 – Interlocking Device with Guard Locking

Safety distance is the distance between the opening covered by the movable guard and the hazard. The minimum distance is determined using the safety distance calculations given in [2] and ISO 13855 [9]. This calculation uses a ‘hand-speed constant’, called K, to represent the theoretical speed that the average person can achieve when extending their hand straight forward when standing in front of the opening. In North America, K is usually 63 inches/second, or 1600 mm/s. Internationally and in the EU, there are two speeds, 2000 mm/s, used for an approach perpendicular to the plane of the guard, or 1600 mm/second for approaches at 45 degrees or less [9]. 2000 mm/s is used with movable guards, and is approximately equivalent to 79 inches/second. Using the International approach, if the value of Ds is greater than 500 mm when calculated using K = 2 000, then [9] permits the calculation to be done using K = 1 600 instead.

Using the stopping time of the machinery and K, the minimum safety distance can be calculated.

Eq. 1              Ds = K x Ts

Using Equation 1 [2], assume you have a machine that takes 250 ms to stop when the interlock is opened. Inserting the values into the equation gives you a minimum safety distance of:

Example 1             Ds = 63 in/s x 0.250 s = 15.75 inches

Example 2             Ds = 2000 mm/s x 0.250 s = 500 mm

As you can see, the International value of K gives a more conservative value, since 500 mm is approximately 20 inches.

Note that I have not included the ‘Penetration Factor’, Dpf in this calculation. This factor is used with presence sensing safeguarding devices like light curtains, fences, mats, two-hand controls, etc. This factor is not applicable to movable, interlocked guards.

Also important to consider is the amount the guard can be opened before activating the interlock. This will depend on many factors, but for simplicity, consider a hinged gate on an access point. If the guard uses two hinge-pin style switches, you may be able to open the gate a few inches before the switches rotate enough to detect the opening of the guard. In order to determine the opening size, you would slowly open the gate just to the point where the interlock is tripped, and then measure the width of the opening. Using the tables found in [2], [3], [10], or ISO 13857 [12], you can then determine how far the guard must be from the hazards behind it. If that distance is greater than what is available, you could remove one hinge-pin switch, and replace it with another type mounted on the post opposite the hinges. This could be a keyed interlock like Photo 3, or a non-contact device like Photo 5. This would reduce the opening width at the point of detection, and thereby reduce the safety distance behind the guard. But what if that is still not good enough?

If you have to install the guard closer to the hazard than the minimum safety distance, locking the guard closed and monitoring the stand-still of the machine allows you to ignore the safety distance requirement because the guard cannot be opened until the machinery is at a standstill, or in a safe state.

Guard locking devices can be mechanical, electromagnetic, or any other type that prevents the guard from opening. The guard locking device is only released when the machine has been made safe.

There are many types of safety-rated stand-still monitoring devices available now, and many variable-frequency drives and servo drive systems are available with safety-rated stand-still monitoring.

Environment, failure modes and fault exclusion

Every device has failure modes. The correct selection of the device starts with understanding the physical environment to which the device will be exposed. This means understanding the temperature, humidity, dust/abrasives exposure, chemical exposures, and mechanical shock and vibration exposures in the application. Selecting a delicate reed switch for use in a high-vibration, high-shock environment is a recipe for failure, just as selecting a mechanical switch in a dusty, damp, corrosive environment will also lead to premature failure.

Example of a non-contact interlocking device
Photo 5 – JOKAB EDEN Interlock System

Interlock device manufacturers have a variety of non-contact interlocking devices available today that use coded RF signals or RF ID technologies to ensure that the interlock cannot be defeated by simple measures, like taping a magnet to a reed switch. The Jokab EDEN system is one example of a system like this that also exhibits IP65 level resistance to moisture and dust. Note that systems like this include a safety monitoring device and the system as a whole can meet Control Reliable or Category 3 / 4 architectural requirements when a simple interlock switch could not.

The device standards do provide some guidance in making these selections, but it’s pretty general.

Fault Exclusion

Fault exclusion is another key concept that needs to be understood. Fault exclusion holds that failure modes that have an exceedingly low probability of occurring during the lifetime of the product can be excluded from consideration. This can apply to electrical or mechanical failures. Here’s the catch: Fault exclusion is not permitted under any North American standards at the moment. Designs based on the North American control reliability standards cannot take advantage of fault exclusions. Designs based on the International and EU standards can use fault exclusion, but be aware that significant documentation supporting the exclusion of each fault is needed.

Defeat resistance

Diagram showing one method of preventing interlock defeat.
Figure 6 – Preventing Defeat

The North American standards require that the devices chosen for safety-related interlocks be defeat-resistant, meaning they cannot be easily fooled with a cable-tie, a scrap of metal or a piece of tape.

Figure 6 [7, Fig. 10] shows a key-operated switch, like the Schmersal AZ15, installed with a cover that is intended to further guard against defeat. The key, sometimes called a ‘tongue’, used with the switch prevents defeat using a flat piece of metal or a knife blade. The cover prevents direct access to the interlocking device itself. Use of tamper-resistant hardware will further reduce the likelihood that someone can remove the key and insert it into the switch, bypassing the guard.

Inner-Tite tamper resistance fasteners
Photo 6 – Tamper-resistant fasteners

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The International and EU standards do not require the devices to be inherently defeat resistant, which means that you can use “safety-rated” limit switches with roller-cam actuators, for example. However, as a designer, you are required to consider all reasonably foreseeable failure modes, and that includes intentional defeat. If the interlocking devices are easily accessible, then you must select defeat-resistant devices and install them with tamper-resistant hardware to cover these failure modes.

Photo 6 shows one type of tamper resistant fasteners made by Inner-Tite [13]. Photo 7 shows fasteners with uniquely keyed key ways made by Bryce Fastener [14], and Photo 8 shows more traditional tamperproof fasteners from the Tamperproof Screw Company [15]. Using fasteners like these will result in the highest level of security in a threaded fastener. There are many different designs available from a wide variety of manufacturers.

Bryce Key-Rex tamper-resistant fasteners
Photo 7 – Keyed Tamper-Resistant Fasteners
Tamper proof screws made by the Tamperproof Screw Company
Photo 8 – Tamper proof screws

Almost any interlocking device can be bypassed by a knowledgeable person using wire and the right tools. This type of defeat is not generally considered, as the degree of knowledge required is greater than that possessed by “normal” users.

How to select the right device

When selecting an interlocking device, start by looking at the environment in which the device will be located. Is it dry? Is it wet (i.e., with cutting fluid, oil, water, etc.)? Is it abrasive (dusty, sandy, chips, etc.)? Is it indoors or outdoors and subject to wide temperature variations?

Is there a product standard that defines the type of interlock you are designing? An example of this is the interlock types in ANSI B151.1 [4] for plastic injection moulding machines. There may be restrictions on the type of devices that are suitable based on the requirements in the standard.

Consider integration requirements with the controls. Is the interlock purely mechanical? Is it integrated with the electrical system? Do you require guard locking capability? Do you require defeat resistance? What about device monitoring or annunciation?

Once you can answer these questions, you will have narrowed down your selections considerably. The final question is: What brand is preferred? Go to your preferred supplier’s catalogues and make a selection that fits with the answers to the previous questions.

The next stage is to integrate the device(s) into the controls, using whichever control reliability standard you need to meet. That is the subject for a series of articles!

References

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[1] Safety of machinery – General principles for design – Risk assessment and risk reduction, ISO Standard 12100, Edition 1, 2010

[2] Safeguarding of Machinery, CSA Standard Z432, 2004 (R2009)

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[3] Industrial Robots and Robot Systems – General Safety Requirements, CSA Standard Z434, 2003 (R2008)

[4] Safety of machinery — Safety-related parts of control systems — Part 1: General principles for design, ISO Standard 13849-1, 2006

[5] Safety of machinery – Functional safety of safety-related electrical, electronic and programmable electronic control systems, IEC Standard 62061, Edition 1, 2005

[6] Functional safety of electrical/electronic/programmable electronic safety-related systems (Seven Parts), IEC Standard 61508-X

[7] Safety of machinery — Interlocking devices associated with guards — Principles for design and selection, ISO Standard 14119, 1998

[8] American National Standard for Machines, General Safety Requirements Common to ANSI B11 Machines, ANSI Standard B11, 2008
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[9] Safety of machinery — Positioning of safeguards with respect to the approach speeds of parts of the human body, ISO 13855, 2010

[10] American National Standard for Machine Tools – Performance Criteria for Safeguarding, ANSI B11.19, 2003

[11] Safety of machinery — Guards — General requirements for the design and construction of fixed and movable guards, ISO 14120. 2002

[12] Safety of machinery – Safety distances to prevent hazard zones being reached by upper and lower limbs, ISO 13857. 2008.

[13] Inner-Tite Corp. home page. (2012). Available: http://www.inner-tite.com/

[14] Bryce Fastener, Inc. home page. (2012). Available: http://www.brycefastener.com/

[15] Tamperproof Screw Co., Inc., home page. (2013). Available: http://www.tamperproof.com