Machinery Safety 101

Using Form C relays in safety circuits

I recently had a dis­cus­sion with a col­league 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
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.

Category 3 Architecture Logic Block Diagram
ISO 13849 – 1, Fig­ure 5, 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. 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.

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.

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.

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.

Form C DPDT Relay. image [6]

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

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.

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.

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

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

Typ­ic­al relay name­plate with Class A mark­ing. image: Omron [13]

For more depth on force-guide 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]


[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].

[12] “Safety Cir­cuit Examples of Safety Com­pon­ents | Tech­nic­al Guide | Aus­tralia | Omron IA”,, 2019. [Online]. Avail­able: [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].


Thanks are due to the fol­low­ing people: my anonym­ous col­league who ori­gin­ally asked the ques­tion, Mr. Les Young, and Mr. Anthony Ker­stens.

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