Voltage leak on Allen Bradley A/C relay output modules

[Jasondelane]

I can't seem to find the answer to this in search. When I'm troubleshooting a faulty 120v valve that is controlled by an A/C relay output module on a Controllogix rack, why do I get 72v when the contact is not closed? Is this the normal "off" condition for A/C relay output?

 

[Jasondelane]

Module is an 1756-OW16I

 

[Bit_Bucket_07]

I can't seem to find the answer to this in search. When I'm troubleshooting a faulty 120v valve that is controlled by an A/C relay output module on a Controllogix rack, why do I get 72v when the contact is not closed? Is this the normal "off" condition for A/C relay output?

Relay output modules have surge suppressors across the contacts, so although they don't exhibit as much leakage current as does a triac output, some leakage is normal.

 

[Jasondelane]

Relay output modules have surge suppressors across the contacts, so although they don't exhibit as much leakage current as does a triac output, some leakage is normal.

So what I'm measuring is the voltage across the surge suppressor. When a load is present it doesn't draw enough current to actuate anything but it's not 0.

 

[Bit_Bucket_07]

So what I'm measuring is the voltage across the surge suppressor. When a load is present it doesn't draw enough current to actuate anything but it's not 0.

That surprises a lot of electricians, because they assume a "dry contact" in a relay output module, but that's not really the case. Only a high impedance meter will read a voltage from the bleed current of the surge suppressors.

 

[GaryS]

if you are seeing 70 volts on an output that's off then you do not have a load on it any lamp or solenoid would pull it down
your volt meter will only load the output by a few micro amps. most meters in us today have an input impedance of a 100 meg ohms
check the wiring to the solenoid for an open

 

[Maxkling]

High impedance DMM’s will show the leakage and as stated above will not draw it down. You can use a solenoid meter like a Wiggy, just be careful with them.

 

[Jasondelane]

if you are seeing 70 volts on an output that's off then you do not have a load on it any lamp or solenoid would pull it down
your volt meter will only load the output by a few micro amps. most meters in us today have an input impedance of a 100 meg ohms
check the wiring to the solenoid for an open

This makes sense. Like I said, we were troubleshooting a faulty valve, and it seems to be a broken connection between the output and the valve. I've noticed this on other modules around the plant and it's almost always 72v. I'm using a Fluke DMM.

So in the future, if there is a load present on the output, I should be reading 0 volts. If there is voltage present, it means there is no load, so I should be checking fuses, terminals, and the load itself.

This will be helpful in the future with troubleshooting. Thanks!

 

[GaryS]

Trouble shooting 101
now your getting it

 

[Jasondelane]

Trouble shooting 101
now your getting it

I've been doing this stuff for 20 years and never thought to ask the question until now.

 

[GaryS]

I have been doing this for 40+ years
I don't mean to put anybody down but when trouble shooting always start with the basics
and then expand

 

[ndzied1]

High impedance DMM’s will show the leakage and as stated above will not draw it down. You can use a solenoid meter like a Wiggy, just be careful with them.

In. Pinch you can make a tester from a pilot light with an incandescent bulb. It should not light up with the output off.