AB safety relay circuit troubleshooting

unsaint32

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Oct 2012
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My apologies in advance for a newbie question.. For a typical AB safety relay with S11 to S12 for CH A, and S21 to S22 for CH B, what is the best way to check the safety circuit continuities? I think the best way is to first place the DC voltameter probes across S11 and S12. If there is 12V there, keep the probe on S11 and move the probe on the S12 back to each estop switch until reading 0 volt. Is there a better way? Thanks.
 
You'll want to put your black probe on your DC common terminal and then use the red to check for voltage (most likely 24VDC) going from S11 to S12 and/or S21 to S22. If you just measure voltage across S11 and S12 you probably won't actually see anything. I think that your getting your continuity and voltage measurements a little mixed up. You also won't want to do continuity measurements with the safety relay powered up. Stick with the voltage as it is simpler.
 
Most Safety relays generate their own supply for the two channels so do not rely on the common for the power supply, I suggest you power down then check the continuity of the two channels. There will possibly be a small resistance depending on the cable lengths and type of safety contacts.
On some AB relays they also have fused channels the fuses are under the front cover and look like small round brown or black potted fuses and fitted into sockets. If not then it is usual to fuse them externally with a low current fuse.
 
As keshik stated if you are trying to find a single contact failure on an E-stop that is dropping out one channel, Power down and test continuity. I use to bring each E-stop
button back to main panel on a 4 wire cable to terminate in a row for trouble shooting this.
Now I use a Safety relay for each E-stop button set on auto reset and have each safety relay wired to the Master Safety relay that has a manual reset. I use the AUX. contacts on the E-stop relays to the PLC for identifying conditions on the HMI costs a little more but for me it is worth it.
 
As I recall, S11 outputs a pulse train and S12 receives it. The redundant channel being S21 and S22, I believe. Swap one of those two wires and you will have issues as the frequencies are not the same. If you have continuity, safe 1 and 2 LED's will light up, unless you have it set to manual reset, then you will have to make sure S34 makes then breaks. Do you have expansion modules tied to this safety relay?
 
As I recall, S11 outputs a pulse train and S12 receives it. The redundant channel being S21 and S22, I believe. Swap one of those two wires and you will have issues as the frequencies are not the same.

It depends on the wiring configuration.
There are many ways to wire the very same relay. Look at the wiring configuration examples for the AB relays. Try to identify which one your using. I have one set of machines here that use similar components to one another but are wired differently. On one machine, 24vdc can be observed channel to channel through the entire safety circuit, One channel is a positive signal, one channel is a negative signal. Simply metering point to point makes troubleshooting easy. On the second machine, each safety channel is using a positive signal. So,I test 24vdc from channel to a known 0vdc point to troubleshoot. Also very easy. But you do need to know how its wired.


Swapping wires could cause a short, depending on your wiring.
 
I bought a machine from an OEM with AB safety relays. The way they wired it up, they would come off the relay, and go to terminal block, then go to the first estop button, then come back to the another terminal block, then go to the 2nd estop, and come back to the last terminal block. So when checking for continuity, you would literally just move the red lead down the ladder of terminal blocks until you lost voltage.

I'm unsure why, but I had an AB relay where channel 1 was 14 V and channel 2 was 12 V or something like that? Weird, but that's the way the module worked.
 
I'm unsure why, but I had an AB relay where channel 1 was 14 V and channel 2 was 12 V or something like that

The signals are actually 24V DC, but safety relays use pulse signals to check for cross-wiring faults. Channel 1 is a particular pulse speed, while Channel 2 is a different speed, and the input side of the device looks for continuous or wrong-speed signals.

When you put an ordinary voltmeter on that signal, it appears to be a lower average voltage, based on the duty cycle of the pulse train.
 
The safety relays used a 24V pulse to check they receiving their own 24V back and not another source. A meter will read this at ~12V
 
I just had an issue with one today and learned something else, that most of you probably already know. When you reset it, both channels must reset at the same time. I opened one channel, tried to reset it, and it wouldn't reset. I had to then open both channels, then close them at the same time. Ashamedly, it took me half an hour to figure this out. o_O
 
Yup ! Simultaneous operation is often a requirement, so you can tell the difference between a wire falling off a terminal and the actual operation of a pushbutton.

Using programmable safety devices like the Banner XS26 with configurable channels and features has taught me a lot about the under-the-hood operational principles of less-programmable safety relays, including things like cross-checking and simultaneous operation.
 
The idea of safety relays is that the channel relays have positive guided contacts and are monitored, on dual channel systems both safety loops must close within a few milliseconds of each other or it will not reset, a common problem in wet areas are water on the contacts or junction boxes this can cause one channel to close before the other this usually indicated by one of the channel LED's to creep up to full brightness (had this many times), also if one channel stays closed and one open, closing the other channel will not allow the relay to reset. so in other words: both channels must open and both channels must close within a time limit or it is considered an error.
 

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