about the internal wiring of a typical safety relay.

My understanding is that the S13 & S14 circuit has 24V+, and S23 & S24 circuit has 0V. And the K1 ... Kn coils energize only when the two circuits are closed. So, here are my questions.

Q1) I get 24V across S13 & S23 as I am supposed to, but why can't I get 24V across S13 terminal and the source GND?

Q2) Between S13 & S14, which is the source side?

Thanks.

S13& S14 are termed channel 1
S23 & S24 are termed channel 2

These channels are not referenced to ground. They supply a voltage from their own internal supply and ground reference (the easiest way to view these channels is as supplies to volt free contacts - ie your e-stop p/buttons). S13 supplies a signal to your e-stop circuit which if made returns that signal back to S14 - Ditto S23 & S24 only when both channels are ok do the output relays fire (these channels have to be made together within 250mS)

This is a very simplified explanation of a very basic safety relay

Check out and Google Pilz safety relays for further explanation.

Steve

Let's state that any Sxy marked contact of any Safety Relay does not carry any "user measurable" electrical circuitry "quantities" unless internal relay contacts "fed" by user circuitry to be "safely controlled".
Whatever you are DMM "measuring" between S marked Input contacts is not what the DMM displays and you cannot approach troubleshooting the way you'd do it on a "generic" electrical circuit.
The S marked Input contacts of any Safety Relay are actually carrying manufacturer specific "pulse forms" some of which will never be "detected" by a common DMM measuring approach; I have replaced quite a few burnt up relays after DMM troubleshooting ("I don't know what happened?...I put my meter on the cable end and there was no power...It never sparked when I cut it or shorted the wiring...").
You will have to go "cold" when troubleshooting/verifying safety circuitry(no power within the safety system); circuit continuity ("Zero" Resistance) is the only measurable circuit value that matters.

dmargineau said:

You will have to go "cold" when troubleshooting/verifying safety circuitry(no power within the safety system); circuit continuity ("Zero" Resistance) is the only measurable circuit value that matters.

Click to expand...

I cannot read voltage between s13 & s23 in some safety relays?

By "cold," you mean I must shut down the system and use ohmmeter?

Yes.
Turn the Safety System power off (Safety Relay supply power); use an Ohmmeter and verify the continuity of the circuits connected to the S marked Safety Relay contacts.