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Public Review: CSA Z1006 Management of Work in Confined Spaces.

2015 July 30
by Doug Nix

CSA Z1006 Management of Work in Confined Spaces  is now available for Public Review. 

As new standards are developed, and existing standards are revised, they are made available for public review and comment. If you are interested in Confined Space Entry, and would like an opportuinity to review the proposed changes to the standard, read on!

Management of work in confined spaces, New Edition | CSA Public Review System

This Standard specifies requirements for and provides guidance on the activities required to manage all aspects of work in confined spaces in accordance with the Plan-Do-Check-Act cycle and management system principles such as those set out in CSA Z1000-15, Occupational health and safety management. This Standard specifies requirements concerning management commitment, leadership, and participation, assignment of roles and responsibilities, identification of confined spaces, identification of hazards, risk assessment, selection and application of controls, design considerations, training, monitoring and measurement, emergency response, documentation, internal audits, and management reviews.

Five informative Annexes provide guidance on implementing this Standard’s normative requirements, including sample forms that can be customized for the specific needs of the user.

This Standard provides:

(a)   An overview of the steps an organization needs to take to establish and maintain an effective confined space management program;

(b)   Safety information for workers entering confined spaces and for persons responsible for ensuring the safety of such workers; and

(c)   Requirements for confined space emergency preparedness and rescue.

Clause 4 specifies general requirements for a comprehensive confined space management program.

Clauses 5 to 8 specify requirements for planning and implementing a confined space management program. Clause 5 specifies roles and responsibilities. Clause 6 specifies requirements for hazard identification and risk assessment, development of entry procedures, emergency response planning, and assessment of worker capability for performing assigned duties within a confined space. Clause 7 specifies requirements related to training, Clause 8 specifies requirements related to controls, emergency response activities, and documentation.

Clauses 9 and 10 specify requirements related to incident investigation and analysis, corrective actions, internal audits, and management reviews. These activities can help ensure worker safety and facilitate continual improvement of a confined space management program.

Changes in this edition include:

a)    Improved flow and readability of the document with the inclusion of additional flowcharts and tables and restructuring of content and clauses;

b)    Added two additional informative Annexes to elaborate and provide additional information on rescue planning and atmospheric testing, monitoring and instrumentation;

c)    Updated the document to harmonize with CSA Z1000, Z1001 and Z1002; and

d)    Elaborated on the information on workspace design and modification.

Use the following link to access the draft standard and comment on any issues you may see in the document: Management of work in confined spaces, New Edition | CSA Public Review System

Thanks to Jill Collins at CSA Group for this notification.

Scoring Severity of Injury – Hidden Probabilities

2015 July 21

I’ve been thinking a lot about risk scoring tools and the algorithms that we use. This all started when I was challenged to write an analysis of the problems with the CSA Risk Scoring Tool that you can find in the 2014 version of CSA Z434. That tool is deeply flawed in my opinion, but that is not the topic of this post. If you want to read my analysis, you can download the white paper and the presentation notes for my analysis on the Compliance insight Publications page [1].

Scoring risk can be a tricky thing, especially in the machinery sector. We rarely have much in the way of real-world data to use in the analysis, and so we are left with the opinions of those building the machine as the basis for our evaluation. Severity is usually the first risk parameter to be estimated because it’s seen as the “easy” one – if the characteristics of the hazard are well known. One aspect of severity that is often missed is the probability of a certain severity of injury. We’re NOT talking about how likely it is for someone to be injured here; we’re talking about the most likely degree of injury that will occur when the person interacts with the hazard. Ok, let’s look at this another way. Let’s call Severity “Se”, any specific injury “I”, and the probability of any specific injury “Ps”. We can then write a short equation to describe this parameter:

Se f (I,Ps)

Since we want there to be a possibility of no injury, we should probably relate these parameters as a product:

Se = I x Ps

Ok, so what? What this equation says is: the Severity (Se) of any given injury (I), is the product of the specific type of injury and the probability of that injury. More simply yet, you could say that you should be considering the most likely type of injury that you think will occur when a person interacts with the hazard. Consider this example: A worker enters a robotic work cell to change the weld tips on the welding gun the robot uses. This task has to be done about once every two days. The entry gate is interlocked, and the robot was locked out before entry. The floor of the work cell has wireways, conduits and piping running across it from the edges of the cell to the various pieces of equipment inside the cell, creating uneven footing and lots of slip and trip hazards. The worker misses his footing and falls. What can you expect for Se in this case?

We know that falls on the same level can lead to fatalities, about 600/year in the USA [2], but that these are mostly in the construction and mining sectors rather than general manufacturing. We also know that broken bones are more likely than same level fatalities. About a million slips and falls per year result in an emergency room visit, and of these, about 5%, or 50,000, result in fractures. Ok, so what do we do with this information? Lets look at at typical severity scale, this one taken from IEC 62061 [3].

Table 1 – Severity (Se) classification [2, Table A.1]

Consequences Severity (Se)
Irreversible: death, losing an eye or arm 4
Irreversible: broken limb(s), losing a finger(s) 3
Reversible: requiring attention from a medical practitioner 2
Reversible: requiring first aid 1

Using Table 1, we might come up with the following list of possible severities of injury. This list is not exhaustive, so feel free to add more.

Table 2 – Potential Injury Severities

Possible Injury Severity (Se)
Fall on same level – Fatality 4
Fall on same level – Broken wrist 3
Fall on same level – Broken collarbone 3
Fall on same level – Torn rotator cuff 2
Fall on same level – Bruises 1
Fall on same level – Head Injury 3
Fall on same level – Head Injury 4

How do we score this using a typical scoring tool? We could add each of these as line items in the risk register, and then assess the probability of each, but that will tend to create huge risk registers with many line items at very low risks. In practice, we decide on what we think is the most likely degree of injury BEFORE we score the risk. This results in a single line item for the hazard, rather than seven as would be the case if we scored each of these potential injuries independently.

We need a probability scale to use in assessing the likelihood of injuries. At the moment, no published scoring tool that I know of has a scale for this, so let’s do the simple thing: Probability (Ps) will be scored from 0-100%, with 100% being a certainty.

Going back to the second equation, what we are really doing is assigning a probability to each of the severities that we think exist, something like this:

Table 3 – Potential Injuries and their Probabilities

Possible Injury (I) Severity (Se) Probability (Ps)
Fall on same level – Fatality 4  0.0075%
Fall on same level – Broken wrist 3  5%
Fall on same level – Broken collarbone 3  5%
Fall on same level – Torn rotator cuff 2  5%
Fall on same level – Bruises 1  90%
Fall on same level – Head Injury 3 1%
Fall on same level – Head Injury 4   0.0075%
Fall on same level – Lacerations to hands 2 90%

The percentages for fatalities and fractures we taken roughly from [1]. Ok, so we can look at a table like this and say that cuts and bruises are the most likely types of injury in this case. We can either decided to group them for the overall risk score, or we can score each individually, resulting in adding two separate line items to the risk register. I’m going to use the other parameters from [2] for this example. In the Example risk Register,

Se = Severity

Pr = Probability of the Hazardous Event

Fr = Frequency and Duration of Exposure

Av = Possibility to Avoid or Limit Harm

The algorithm I am using to evaluate the risk is R = Se x [Pr x (Fr + Av)] [1]. Note that where I have combined the two potential injuries into one line item (Item 1 in the register), I have selected the highest severity of the combined injuries. The less likely severities, and in particular the fatalities, have been ignored.

Table 4 – Example Risk Register

Intrinsic Risk Score Final Risk Score
Item # Life Cycle Stage Task / Activity Se Pr Fr Av Risk Mitigation Se Pr Fr Av Risk
1  Use: Maintenance Changing weld tip on robot welding gun: Slip and fall on same level – Bruises & Lacerations 2  5  4  3  70 Install false floor above conduits, raceways, and cables to eliminate trip hazards. 2 2 4 3 20
2  Use: Maintenance  Changing weld tip on robot welding gun: Slip and fall on same level – Bruises  1  5 4 3  35 Install false floor above conduits, raceways, and cables to eliminate trip hazards. 1 2 4 3 10
3 Use: Maintenance  Changing weld tip on robot welding gun: Slip and fall on same level – Lacerations  2  5 4 3  70 Install false floor above conduits, raceways, and cables to eliminate trip hazards. 2 2 4 3 20

Note that I did not reduce the Se scores in the Final Score, because I have not made changes to the hazards themselves, only to the likelihood of the injury occurring. In all cases we can show a significant risk reduction, but I’m not going to get into risk evaluation (i.e., Is the risk effectively controlled?) in this particular article.

Conclusions

Consideration of the probability of certain kinds of injuries occurring must be considered when estimating risk. This process is largely undocumented, but nevertheless occurs. When risk analysts are considering the severity of injury from any given hazard, this article gives the reader one possible approach than can be used to select the types of injuries most likely to occur before scoring the rest of the risk parameters.

References

[1] D. Nix, ‘Evaluation of Problems and Challenges in CSA Z434-14 Annex DVA Task-Based Risk Assessment Methodology‘, 2015.

[2] National Floor Safety Institute (NFSI), ‘Quick Facts – Slips, Trips, and Falls’, 2015. [Online]. Available: http://nfsi.org/nfsi-research/quick-facts/. [Accessed: 21- Jul- 2015].

[3] ‘Safety of machinery – Functional safety of safety-related electrical, electronic and programmable electronic control systems. IEC 62061.’, International Electrotechnical Commission (IEC), Geneva, 2005.

 

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Presence Sensing Devices – Reaching over sensing fields

2015 May 29
Comments Off on Presence Sensing Devices – Reaching over sensing fields
Cell Elevation View

I recently heard about an application question related to a light curtain where a small gap existed at the top of the sensing field, between the last beam in the field and the surrounding structure of the machine. There was some concern raised about the gap, and whether or not additional guarding might be needed to close the gap. To answer this question, we need to split it into a few smaller pieces that we can deal with using the tools in the standards.

The first piece to consider is the gap at the top of the sensing field. For this part of the analysis, I’m going to assume that the light curtain is a fixed barrier guard, and we’ll analyse the gap based on that idea.

The second piece of the puzzle is the placement of the light curtain, and we’ll look at that separately. Once we we understand the two pieces, we’ll put them together to see if there are any other issues that may need to be addressed.

The Application

For the purpose of this article, I’ve sketched up the following figures to illustrate the ideas in the article. These drawings don’t represent any actual robot cell or application. Note that the light curtain in the sketch is shown with zero safety distance to the robot envelope. This is NEVER permitted.

Cell Elevation View

Figure 1 – Cell Elevation View showing Gap above Light Curtain

 

Cell Plan View

Figure 2 – Cell Plan View

Analyzing The Gap

Light curtains are treated the same way that movable guards are treated, so the answer to this question starts with determining the size of the gap. I’m going to reference two sets of standards in answering this question: CSA and ISO.

Safety Distances for fingers reaching through an opening

Figure 3 – Finger-to-Knuckle Reaching through a Regular Opening [1, C.4]

Z432 Reaching Through Regular Openings

Figure 4 – Arm-up-to-Shoulder Reaching through Regular Opening [1, C.4]

Referenced Standards
CSA Z432 2004 [1] ISO 13857 2008 [2]
Table 3 – Minimum distance from hazard as a function of barrier opening size Table 4 – Reaching through Regular openings
Opening Size (e) Safety Distance (sr) Opening Size (e) Safety Distance (sr)

11.1– 16.0mm [0.376″–0.625″]

Slotted >= 89.0 mm [3.5″]Square >= 66 mm [2.6″] Slot 10 < e <=12Square/Round 10 < e <=12 >= 100 mm>= 80 mm
49.1–132.0 mm [1.876–5.000″] Slotted/Square <= 915.0 mm [36.0″] Slot/Square/Round 40 < e <= 120 mm <= 850 mm

The first thing to notice is that CSA and ISO use slightly different opening sizes (e) and safety distances (sr). These differences have their origin in slightly different anthropometric data used to develop the tables. In both cases, the maximum value for e defines the largest opening permitted without additional guarding.

Let’s look at the application to see if the gap between the top-most beam and the edge of the physical guard falls into the bands defined for e.

Cell Elevation Close Up

Figure 5 – Cell Elevation Close-Up

Based on the sketches of the application, we have a problem: The gap shown above the light curtain is right at the edge of the robot envelope, i.e., the danger zone. We are going to have to either, a) Move the fence back 915 mm to get the necessary safety distance or, b) close the gap off completely, either with hard guarding, or by extending the light curtain to close the gap.

Knowing the size of the gap, we can now decide if the gap should be reduced, or the light curtain moved or enlarged. Since light curtains run about $125/linear inch, adding an additional plate to reduce the size of the gap is likely the most cost effective choice. We also need to know the distance from the top-most beam of the light curtain to the hazard behind the guard. If that distance is less than 915/850 mm, then we have another problem, since the guarding is already too close to the hazard.

Analyzing the Light Curtain

The light curtain positioning is driven by the stopping performance of the machine. Again, let’s reference both CSA and ISO for the relevant calculations.

Referenced Standards
CSA Z432 2004 ISO 13855 2005 [3]
5.1 Overall system stopping performance
The overall system stopping performance comprises at least two phases.Thetwophasesare linked by Equation (1):

T = t1 + t2                             (1)

where
T is the overall system stopping performance;
t1 is the maximum time between the occurrence of the actuation of the safeguard and the output signal achieving the OFF-state;
t2 is the stopping time, which is the maximum time required to terminate the hazardous machine function after the output signal from the safeguard achieves the OFF-state. The response time of the control system of the machine shall be included in t2.

t1 and t2 are influenced by various factors, e.g. temperature, switching time of valves, ageing of components.

t1 and t2 are functions of the safeguard and the machine, respectively, and are determined by design and evaluated by measurement. The evaluation of these two values shall include the uncertainties resulting from the measurements, calculations and/or construction.

Clause 10.11 – Safeguarding device safety distanceThecalculationforminimum safe distance between a safeguarding device and the danger zone of a machine shall be as follows:

S = [K (Ts + Tc + Tr + Tbm)] + Dpf

where
Ds = minimum safe distance between the safeguarding device and the hazard

K = speed constant: 1.6 m/s (63 in/s) minimum, based on the movement being the hand/arm only and the body being stationary.
Note: A greater value may be required in specific applications and when body motion must also be considered.
Ts = worst stopping time of the machine/equipment

Tc = worst stopping time of the control system

Tr = response time of the safeguarding device, including its interface
Note: Tr for interlocked barrier may include a delay due to actuation. This delay may result in Tr being a deduct (negative value).

Note: Ts + Tc + Tr are usually measured by a stop-time measuring device if unknown.

Tbm = additional stopping time allowed by the brake monitor before it detects stop-time deterioration beyond the end users’ predetermined limits. (For part revolution presses only.)

Dpf = maximum travel towards the hazard within the presence-sensing safeguarding device’s (PSSD) field that may occur before a stop is signaled. Depth penetration factors will change depending on the type of device and application. See Figure 5 for specific values. (If applicable, based on the style of safety device.)

Clause 6.2.3 – Electro-sensitive protective equipment employing active opto-electronic protective devices with a sensor detection capability of ? 40 mm  in diameter

6.2.3.1 Calculation

The minimum distance, S, in millimetres, from the detection zone to the hazard zone shall not be less than that calculated using Equation (2):

S = (K x T ) + C                             (2)

where

K = 2 000 mm/s;

C = 8 (d – 14), but not less than 0;

d is the sensor detection capability of the device, in millimetres (mm).

[Author’s Note – T comes from 5.1, above]

Then

S = (2 000 x T ) + 8(d-14)               (3)

Equation (3) applies to all minimum distances of S up to and including 500 mm. The minimum value of S shall be 100 mm.

Where the values for S, calculated using Equation (3), exceed 500 mm, Equation (4) can be used. In this case, the minimum value of S shall be 500 mm.

S = (K x T ) + C                          (2)

where

K = 1 600 mm/s;

C = 8 (d – 14), but not less than 0;

d is the sensor detection capability of the device, in millimetres (mm).

Then

S = (1 600 x T ) + 8(d – 14)

ISO 13855 Fig. 3 a) Normal Approach

Figure 6 – ISO 13855 Fig. 3 a) Normal Approach

Key

1 hazard zone

2 detection zone

3 fixed guard

S minimum distance

a Direction of approach

The two calculation methods shown above are essentially the same, with the primary difference being the value of K, the “hand-speed constant”. ISO uses a higher value of K for light curtain installations where the field is vertical, or angled as low as 45º. If the calculated value of S is >500 mm, then the value of K is reduced to 1600 mm/s. Using the higher value of K for a North American installation is not wrong, and will result in a more conservative installation result. Use of 1 600 mm/s for machines going into international markets is wrong if S is <500 mm when calculated using 2 000 mm/s.

Let’s assume some values so we can do a representative calculation:

Stopping Time of the system (T) = 265 ms [0.265 s]

Light curtain resolution (d) = 30 mm [1.2″]

Calculating Dpf

Dpf = 8 x (d – 14) = 8 x (30 – 14) = 128

Using K = 2 000 mm/s

S = (2000 x 0.265) + 128 = 658 mm

Since applications where S > 500 mm can be recalculated using K = 1600 mm/s

S = (1 600 x 0.265) + 128 = 552 mm

So, from the above calculation we can see that the distance from the plane of the light curtain to the edge of the robot envelope (i.e., the danger zone) must be at least 552 mm [21.75″]. That distance is enough that some people might be able to stand between the light curtain field and the fixture in the cell, so we should probably add a horizontal light curtain to protect against that possibility. See Figure 7.

Figure 7 - Vertical Light Curtain with Horizontal segment

Figure 7 – Vertical Light Curtain with Horizontal segment [1, Fig. B.15 (c)]

Another alternative to adding a horizontal section is to slope the light curtain field, so that the plane of the light curtain is at 45 degrees above the horizontal, with the highest beam as far away from the hazard as possible. See Figure 8.

Figure 8 - Sloped light curtain installation [1, CSA Z432 Fig B.15 (c)]

Figure 8 – Sloped light curtain installation [1, CSA Z432 Fig B.15 (c)]

This type of installation avoids the need to replace the existing light curtain, as long as the field depth is enough to meet the calculated Ds.

The field could also be laid horizontally, with no vertical component. This will change the Dpf calculation as highlighted by the note in Figure 8. Dpf for a horizontal field is calculated using the following equation:

Dpf = 1 200 mm [48″]

therefore

S = (1 600 x 0.265) + 1200 = 1 624 mm

Note also that there is a height restriction placed on horizontal devices based on the object resolution as well, so the 0.3 m maximum height may not apply to an exclusively horizontal application. Note that ISO 13855 allows H a maximum value of 1 000 mm, rather than cutting the value off at 990 mm as done in CSA Z432. Using either the 14 mm or the 30 mm resolution curtains yields a minimum height of 0 mm and a maximum of 990 mm (CSA) or 1 000 mm (ISO). Note that the 3rd Edition of CSA Z432 is likely to harmonize these distances with the ISO calculations, eliminating these differences.

Also note that field heights where H > 300 mm may require additional safeguards in conjunction with the Presence-Sensing Safeguarding Device (PSSD) field.

Figure 8 - Calculating "H" [1, Fig. B.15 (g)]

Figure 8 – Calculating “H” [1, Fig. B.15 (g)]

Going back to our original vertical field installation, there is one more option that could be considered: Reduce the object resolution of the light curtain. If we go down to the smallest object resolution typically available, 14 mm , the calculation looks like this:

Dpf = 8 x (14-14) = 0

S = (2 000 x 0.265) + 0 = 530 mm

Since S > 500,

S = (1 600 x 0.265) + 0 = 424 mm [16.7″]

While we have substantially reduced the safety distance, it looks like we will still need the horizontal light curtain to ensure that no one can stand behind the curtain without being detected.

If the design of the machinery allows, it might be possible to reduce the stopping time of the machine. If you can reduce the stopping time, you will be able to shorten the safety distance required. Note that the safety distance can never go to zero, and can never be less than that determined by the object resolution applied to the reaching-through tables. In this case, a 14 mm opening results in an 89 mm [3.5″] minimum safety distance (CSA). Since the stopping time of the machine can never be zero, 89 mm works out to a stopping time of 44.5 ms using K=2 000 mm/s, or 55.6 ms if K= 1 600 mm/s. Very few machines can stop this quickly.

The calculated safety distance is about half of the safety distance required for the gap, at 915 mm. Clearly, closing the gap with the light curtain or hard guarding will be preferable to moving the fence away from the danger zone by 915 mm.

Here’s one more figure to help illustrate these ideas.

Z432 Figure B.14 a

Figure 9 – CSA Z432 Figure B.15 a)

Figure 9 shows the difference between the reaching-through or reaching-over light curtain applications. Notice that without a restricting guard above the curtain as we have in our example, the Dpf value goes out to 1 200 mm [48″], rather than the 915 mm value used in our example.

The lower figures show light fence applications, where two or three beams are used, rather than a full coverage light curtain.

Summary

Here are some of the more important considerations:
1) Is the field of the light curtain placed correctly, based on the stopping performance of the machine?
2) What is the object resolution of the sensing field? This dimension may be used to assess the size of the “openings” in the field if this becomes relevant.
3) What is the height of the lowest and highest beams or the edges of the sensing field?
4) What are the dimensions of the gap above the field of the curtain, and the distance from the opening to the closes hazard?

Acknowledgements

I’d like to acknowledge my colleague, Christian Bidner, who suggested the idea for this article based on a real-world application he had seen. Christian works for OMRON/STI in their Toronto office.

References

[1]     Safeguarding of Machinery. CSA Z432. Canadian Standards Association (CSA).  Toronto. 2004.

[2]     Safety of machinery – Safety distances to prevent hazard zones being reached by upper and lower limbs. ISO 13857.International Organization for Standardization (ISO). Geneva. 2008.

[3]     Safety of machinery – Positioning of safeguards with respect to the approach speeds of parts of the human body. ISo 13855. International Organization for Standardization (ISO). Geneva. 2010.

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