I've been trying to understand how these safety relays from Weidmuller work. But since I can't find any useful resource on the internet, it has remained unclear for me. Here is the link to data sheet: SIL3 RELAYS

This is the internal diagram:

Internal block diagram

(Source: Weidmüller)

The datasheet says these relays are used in areas that require functionally safe deactivation or activation. But how?

  • Are they different from a normal relay?
  • And why some contacts are connected in parallel and some in series?
  • And then what are test inputs? (The data sheet says that these inputs are for testing relay contacts, but how?)
  • 2
    \$\begingroup\$ The relays are tested for very low failure rates, the contacts are designed so that they can be completely disconnected (breaking both sides of a power connection instead of just one), the series contacts are safety interlocks, both inputs must be activated to turn the device on, the parallel are to connect the neutral regardless. \$\endgroup\$ – Passerby Feb 11 '17 at 7:12
  • 1
    \$\begingroup\$ Without reading much detail in the manual I tell two things: It provides the series relays for safe turn off and the parallel relays for safe holding. Imagine one of the paired relays fails: in the former case a should-turn-off-but-holds failure can be prevented by the other relay opening anyway. In the latter case, when holding is required the other relay will cover the relay-releases-when-it-should-hold case. Don't miss the manual package linked at the bottom of the table. \$\endgroup\$ – try-catch-finally Feb 11 '17 at 9:01
  • \$\begingroup\$ I hadn't mentioned the manual package!thank u :) @try-catch-finally \$\endgroup\$ – MINA ASHTIANI Feb 11 '17 at 12:51

This relay provides the series relays for safe turn off and the parallel relays for safe turn on (holding). This mitigates single point of failure errors.

Relays have two failure modes:

  1. contact is closed while it should be opened
  2. contact is open while it should close

This might be caused by mechanical defects or welded or burnt contacts.

When reliable turn off or turn on is required such as in industrial (EX) or medical environments, these relay types provide higher security and reliability.

From the manual:

It is used for safely energising (ETS = energised to safe) and de-energising (DTS = de-energised to safe) system components in process industry applications.


  • Are they different from a normal relay?

Yes, in that they provide testing and higher security.

From the manual:

There are 2 separate outputs available, each with 2 contacts in a row (DTS) or parallel to each other (ETS) for safely energising and de-energising the components. In accordance with safety procedures, only one output circuit can be used at a time.

Case 1, DTS: Two relays in series will be turned off together. If one relay fails and stays closed, the other relay will still open. Thus the final state is open.

Case 2, ETS: Two relays in parallel will be turned on together. If one relay fails and stays open, the other relay will still close. Thus the final state is closed.

  • And why some contacts are connected in parallel and some in series?

To reliably turn on or reliably turn off.


  • And then what are test inputs? (The data sheet says that these inputs are for testing relay contacts, but how?)

The three test inputs are used to test the indvidual relays. When applying the test signals according to the iunstruction in 5.1 Functional check a technican can observe the correct or incorrect operation of both relays.

When applying test signals across X1 & X3 and X2 & X3 or X1, X2 & X3 the relays can be switched individually and all together.

See the diagram and logic table:

Internal diagram (Source: Weidmüller)

(Source: Weidmüller)

Logic table (Source: Weidmüller)

(Source: Weidmüller)

This allows forcing failure modes which can then be determined by measuring resistance across the output contacts. The The signal inputs A1 and A2 are not supposed to be used while testing.

See the manual for more details.

  • \$\begingroup\$ Thank you very much for such a useful and detailed answer :) \$\endgroup\$ – MINA ASHTIANI Feb 11 '17 at 12:41

If you have an application requiring safety devices you definitely want to have someone experienced look at it, especially if it requires SIL3 and is a machine people will buy or you will use. Just using a safety component is no guarantee unless you know how to use it. If sold in Europe you need to do that as parts of CE compliance. If not and you were just curious, ignore my reply :)

In "short" there's a lot of work to do, starting with the hazard assessment. The level of hazard will determine what your safely interlock needs to perform at (nothing, SIL1, 2, 3), with lower probabilities of dangerous failure as you go up in level. Your devices, cycle counts, design topology, common cause failures, etc. will all be accounted for in calculating the probability of a dangerous failure of your safety devices. Look up iec 62061 as a starting point. It gets overwhelming quickly if you have not done it before so find tutorials and spend time reading. Always remember the hazard assessment needs to be done as if the safety devices are not present since you are trying to determine what the safety devices need to be, if any. That is a common trap for people. Always do it with multiple coworkers who are familiar with the machine and document it. I prefer to use the iec 62061 hazard assessment method since it is more granular but a simpler standard that is parallel to it would be iso 13849. Either can be used. Do a Google search for "risk assessment introduction seimens ag" as an example. Manufactures will provide guidance also in hopes you will use their products in yours. From experience, I recommend Euchner or SICK. They have reps everywhere and are very helpful. Euchner will also, for a fee, do an audit of your design for you. SICK has a lot of great tutorials also. Buy through their distributors if you are an OEM. Retail prices are insane.

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