Machinery Safety 101

Using Form C relays in safety circuits

I recently had a dis­cus­sion with a col­league who wanted to know if it was OK for a design to include a form C relay in an e‑stop cir­cuit. You might recall that e‑stop func­tions are required to meet at least PLc/SIL1 require­ments [1], [2]. It’s import­ant to remem­ber that PLc/SIL1 can be met using Cat­egory 1, 2, or 3 archi­tec­tures. See [1, Fig. 5] below.

ISO 13849-1 Figure 5 showing the relationship between PLs, Categories, DC and MTTFD.
Rela­tion­ship between Cat­egor­ies, DC, MTTFd and PL [1, Fig. 5] 

The Short Answer

YES, you can, but you need to under­stand the spe­cial require­ments that apply. For that, read on!

Component Requirements

Fre­quently design­ers get con­fused about require­ments for com­pon­ents to be used in SRP/CS*. To some degree, com­pon­ent selec­tions are based on the archi­tec­tur­al cat­egory selec­ted (i.e., B, 1, 2, 3, 4), but it’s import­ant to under­stand that, except for Cat­egor­ies B and 1, there are no expli­cit require­ments placed on the com­pon­ents used.

*See the Defin­i­tions at the end of this post.

Category B

In Cat­egory B the com­pon­ent require­ments are lim­ited to their suit­ab­il­ity for the pre­vail­ing cir­cuit con­di­tions. This applies equally to elec­tric­al and flu­id­ic com­pon­ents. Cat­egory B forms the basis for all of the oth­er Cat­egor­ies, so this min­im­um require­ment applies in all designs. Hav­ing said this, no com­pet­ent design­er would do any­thing else.

Category 1

In Cat­egory 1 a new require­ment is added: well-tried com­pon­ents. The idea of well-tried com­pon­ents has not been well defined since EN 954 – 1 was pub­lished in 1995, how­ever, this will change in the next edi­tion of the stand­ard. In the mean­time, the tables of well-tried com­pon­ents in ISO 13849 – 2 [3] give expli­cit examples of well-tried com­pon­ents in mech­an­ic­al, elec­tric­al and flu­id­ic technologies.

The use of well-tried com­pon­ents is the heart of Cat­egory 1 since this cat­egory relies on high-reli­ab­il­ity com­pon­ents to achieve a high­er PL.

Categories 2, 3, and 4

In Cat­egor­ies 2, 3, and 4, addi­tion­al reli­ab­il­ity is based on dia­gnost­ic cap­ab­il­ity and the use of redund­ancy. There is noth­ing that would pre­clude using well-tried com­pon­ents in these cat­egor­ies, how­ever, they are not required.

Contactors and Relays

Simple safety cir­cuits com­monly use con­tact­ors or relays for out­put devices, the “O” devices in the block dia­gram below [1, Fig. 11].

Category 3 Architecture Logic Block Diagram
ISO 13849 – 1, Fig­ure 11, Cat­egory 3 Block Diagram

Remem­ber too that the dashed line used for the “m” links shown above indic­ate that some but not all faults may be detec­ted by the dia­gnostics. In Cat­egory 3, this ranges from DCavg = Low (60% to 90%) to DCavg = Med (90% to 99%).


If we look at an example Cat­egory 3 cir­cuit, below, we can see that there are aux­il­i­ary con­tacts from the out­put con­tact­ors, K1 and K2, con­nec­ted into the reset loop (+24 Vdc to S34) on the safety relay to mon­it­or the devices. 

Example Category 3 safety circuit schematic diagram
Example Cat­egory 3 safety cir­cuit. image: [5]

The mech­an­ic­al design of some power con­tact­ors is such that the aux­il­i­ary con­tact and the power con­tacts are mech­an­ic­ally linked in a way that will force all the con­tacts to trans­fer togeth­er. Sim­il­ar to a flu­id­ic valve, con­tact­ors are only open or closed. In the event that a con­tact welds, the con­tact­or will remain in the closed state. 

Cutaway drawing showing the typical construction of a contactor relay.
image: [3]

As you can see, the basic mech­an­ic­al con­struc­tion is quite simple. The con­tacts are moun­ted to a rigid mech­an­ic­al arma­ture, ensur­ing that they move togeth­er when the solen­oid is energized. 

Two cutaway drawings showing the de-energized and the energized state of a contactor relay.
image: [3]

Beware, how­ever, as not all con­tact­ors are built this way. Check to ensure that the con­tact­or data­sheet shows that the con­tacts are force-guided. An easy way to veri­fy this is to look for IEC 61810 – 3 in the list of stand­ards to which the con­tact­or conforms.

What hap­pens if the cir­cuit con­di­tions are such that a smal­ler relay is preferable?

Control Relays

A Form C relay is also some­times called a “changeover” or “Single Pole-Double throw (SPDT)” relay. 

Schematic diagram of an SPDT Form C contact relay. SPDT stands for single-pole double-throw.
Form C relay schem­at­ic. image: [6]

A single form C con­tact isn’t much good unless you are try­ing to design a sys­tem whose PL only requires PLa or PLb, but adding a second con­tact to this device adds the pos­sib­il­ity of mon­it­or­ing the relay. This is called a “double-pole, double-throw” or DPDT relay.

Schematic diagram of a DPDT Form C relay. DPDT stands for double-pole double-throw.
Form C DPDT Relay. image [6]

This is the con­fig­ur­a­tion com­monly found in con­trol relays that phys­ic­ally look some­thing like this:

Photo of a typical general-purpose DPDT control relay. Construction is visible through the transparent relay casing.
Typ­ic­al DPDT Relay. image: [7]

Depend­ing on how the arma­ture is mech­an­ic­ally linked to the con­tact reeds, it is quite pos­sible to have one con­tact trans­fer and one stay closed for a num­ber of reas­ons. This is a “dan­ger­ous undetect­able (?dd)” fault that will crush the reli­ab­il­ity of your design. It’s for this reas­on that “force-guided” relays were created.

Force-guided relays, on the oth­er hand, have a com­pletely dif­fer­ent design from a con­ven­tion­al con­trol relay. In a force-guided relay, the arma­ture is moved to the tip of the con­tact reeds and as close to the con­tacts as pos­sible. This helps ensure that the con­tacts move togeth­er. Also, each con­tact is enclosed in a well inside the plastic body, which will con­tain the reed if it breaks, and also helps to con­tain any debris cre­ated by heav­ily loaded con­tacts. The draw­ing below shows a force guided relay that has failed, with one of the nor­mally open (NO) con­tacts welded.

Diagram showing the detailed construction of a force-guided relay.
Intern­al dia­gram of a force-guided relay. image [11]

You can also see the loc­a­tion of the arma­ture and the con­tact wells in the dia­gram above.

Photo of an example force-guided relay. Construction is visible through the transparent relay cover.
Example of a force-guided relay. image [8]

The close mech­an­ic­al link­ing of the con­tacts helps to ensure that all the con­tacts trans­fer togeth­er. If one con­tact is used as a mon­it­or­ing con­tact for the relay, there should be a high level of con­fid­ence that the state of the mon­it­or­ing con­tact is the same as the in-cir­cuit con­tacts. This mech­an­ic­al design helps solve the prob­lem with the dan­ger­ous undetect­able fault cre­ated by the design of a stand­ard con­trol relay.

Force-guided relays that meet IEC 61810 [14] will bear the Class A con­tact mark shown below. Class A refers to the for­cible guid­ing of the con­tacts. Class B con­tacts are those found in a stand­ard con­trol relay.

Class "A" contact mark used to denote force-guided relay construction on the label of a control relay.
Class A con­tact mark

The Class A con­tact mark will nor­mally show up on the relay name­plate as shown below.

Example of a control relay label by OMRON showing the device approval marks, the Class "A" contact mark, and the contact arrangement diagram.
Typ­ic­al relay name­plate with Class A mark­ing. image: Omron [13]

For more depth on force-guided relay dif­fer­ences as com­pared to stand­ard con­trol relays, see Elesta’s explainer.

The Answer

If you only need the rel­at­ively small cur­rent hand­ling cap­ab­il­ity offered by a con­trol relay rel­at­ive to a con­tact­or, then there is no reas­on you should not select one for use in your SRP/CS with one caveat: it must be force-guided.

Once you’ve selec­ted the appro­pri­ate device, you can then do the ISO 13849 – 1 ana­lys­is using the com­pon­ent reli­ab­il­ity data provided by the relay man­u­fac­turer. In some cases, such as Cat­egory 4 applic­a­tions, you may want to select two dif­fer­ent relays from dif­fer­ent man­u­fac­tur­ers to help with diversity in the SRP/CS. This will be accoun­ted for in your CCF scoring.

Con­tact­ors are equally good, but are phys­ic­ally lar­ger and often have high­er-power solen­oids, so these aspects may also be con­straints on the design. If you are select­ing a con­tact­or, pick­ing a “safety con­tact­or” makes manu­ally over­rid­ing the solen­oid more dif­fi­cult by pre­vent­ing access to the con­tact­or arma­ture with a small tool or a zip-tie. Safety con­tact­ors also ensure that the arma­ture design ensures that the con­tacts are force-guided, includ­ing the aux­il­i­ary contacts.

If this does­n’t answer your ques­tion in enough depth, feel free to get in touch!


For­cibly Guided Con­tacts

  • com­bin­a­tion of make con­tacts and break con­tacts designed in such a way that it is made sure by mech­an­ic­al means that these make con­tacts and break con­tacts can nev­er be in the closed pos­i­tion sim­ul­tan­eously [12]
  • makes it impossible to close the nor­mally closed and nor­mally open con­tact sim­ul­tan­eously. If a nor­mally closed con­tact becomes wel­ded, it must be impossible for the nor­mally open con­tacts to close when the coil is ener­gized.

    If a nor­mally open con­tact becomes wel­ded, it must be impossible for the nor­mally closed con­tacts to close when the coil is de-ener­gized. Also known as: forced con­tacts, pos­it­ively activ­ated con­tacts, guided con­tacts, and linked con­tacts. [9] See IEC 61810 – 3 [10] and [14].

Form C – A “Form C” con­tact has at least 3 ter­min­als. One ter­min­al is com­mon, with one nor­mally open con­tact and one is nor­mally closed con­tact shar­ing the com­mon. This is also called a “changeover” device because the com­mon con­tact changes from the nor­mally closed pos­i­tion to the nor­mally open pos­i­tion when the coil is ener­gized in a relay or a mag­net­ic field is nearby in a reed switch [4]

safety – related part of a con­trol sys­tem

part of a con­trol sys­tem that responds to safety-related input sig­nals and gen­er­ates safety-related out­put sig­nals

Note 1 to entry: The com­bined safety-related parts of a con­trol sys­tem start at the point where the safety-related input sig­nals are ini­ti­ated (includ­ing, for example, the actu­at­ing cam and the roller of the pos­i­tion switch) and end at the out­put of the power con­trol ele­ments (includ­ing, for example, the main con­tacts of a con­tact­or).

Note 2 to entry: If mon­it­or­ing sys­tems are used for dia­gnostics, they are also con­sidered as SRP/CS. [1, 3.1.1]

Single-Pole Double-Throw (SPST), etc. (SPDT, DPST, and DPDT)

SP and DP refer to single pole and double pole, ST and DT refer to single throw and double throw.

Pole refers to the num­ber of cir­cuits con­trolled by the switch: SP switches con­trol only one elec­tric­al cir­cuit. DP switches con­trol two inde­pend­ent cir­cuits (and act like two identic­al switches that are mech­an­ic­ally linked). Do not con­fuse ‘pole’ with ‘ter­min­al’. The DPST switch, for example, has four ter­min­als, but it is a DP, not a 4P switch.

Throw refers to the extreme pos­i­tion of the actu­at­or: ST switches close a cir­cuit at only one pos­i­tion. The oth­er pos­i­tion of the handle is Off. DT switches close a cir­cuit in the Up pos­i­tion, as well as the Down pos­i­tion (On-On). A DT switch can also have a cen­ter pos­i­tion (fre­quently On-Off-On).

Single pole/throw and double pole/throw switches are by far the most com­mon switches, but triple and quad­ruple con­fig­ur­a­tions are also avail­able. They are com­monly denoted 3PST, 3PDT, 4PDT, etc. [15]


[1] Safety of machinery — Safety-related parts of con­trol sys­tems — Part 1: Gen­er­al prin­ciples for design. ISO 13849 – 1. 2015.

[2] Safety of machinery – Func­tion­al safety of safety-related elec­tric­al, elec­tron­ic and pro­gram­mable elec­tron­ic con­trol sys­tems. IEC 62061. 2005.

[3] “Con­tact­or Con­struc­tion & Oper­at­ing Prin­ciple – Elec­tric­al Engin­eer­ing 123”, Elec­tric­al Engin­eer­ing 123, 2019. [Online]. Avail­able: [Accessed: 27- Oct- 2019].

[4] “Under­stand­ing Form A, Form B, Form C Con­tact Con­fig­ur­a­tion”, Engin­eer­ing and Com­pon­ent Solu­tion For­um – Tech­For­um ? Digi-Key, 2019. [Online]. Avail­able: [Accessed: 27- Oct- 2019].

[5] Rock­well Auto­ma­tion, 52582 – Example Safety Cir­cuits Cat­egor­ies for the SMC. 2019.

[6] Sivaranjith, “What are Con­trol relays?”, Indus­tri­al Auto­ma­tion, PLC Pro­gram­ming, scada & Pid Con­trol Sys­tem, 2017. [Online]. Avail­able: [Accessed: 27- Oct- 2019].

[7] grotedikken, “Mag­n­eet aan/uit – For­um – Cir­cuits Online”,, 2018. [Online]. Avail­able: [Accessed: 27- Oct- 2019].

[8] “HOZ-03463/11 – 001614-220/030.01”,, 2019. [Online]. Avail­able: – 001614-220/030.01/4631011.html. [Accessed: 27- Oct- 2019].

[9] “What are force guided con­tacts?”,, 2019. [Online]. Avail­able: [Accessed: 28- Oct- 2019].

[10] “Dis­tin­guish­ing fea­tures of relays with for­cibly guided con­tacts and ele­ment­ary relays / ELESTA GmbH”,, 2019. [Online]. Avail­able: [Accessed: 28- Oct- 2019].

[11] “Safety Relay: How Does a For­cibly Guided Con­tact Work | FAQ | Singa­pore | Omron IA”,, 2019. [Online]. Avail­able: [Accessed: 28- Oct- 2019].

[12] “IEC 60050 – Inter­na­tion­al Elec­tro­tech­nic­al Vocab­u­lary – Details for IEV num­ber 444 – 04-23: “for­cibly guided con­tacts””,, 2019. [Online]. Avail­able: – 04-23. [Accessed: 29- Oct- 2019].

[13] “Safety Cir­cuit Examples of Safety Com­pon­ents | Tech­nic­al Guide | Aus­tralia | Omron IA”,, 2019. [Online]. Avail­able: [Accessed: 29- Oct- 2019].

[14] “IEC 61810 – 3:2015 | IEC Web­store”,, 2019. [Online]. Avail­able: [Accessed: 30- Oct- 2019].

[15] “SPST, SPDT, DPST, and DPDT Explained – Lit­tel­fuse”,, 2020. [Online]. Avail­able: [Accessed: 13- Jan- 2020].


Thanks are due to the fol­low­ing people: my anonym­ous col­league who ori­gin­ally asked the ques­tion, Mr. Les Young, and Mr. Anthony Ker­stens for their com­ments on this art­icle through LinkedIn.