OT: High and low voltage in one relay

[briancr]

Hi All

Would like to here comments from the members about running high and low voltage through the same relay

See attached pdf

Had a situation where the relay faulted and the high voltage shorted through to the low voltage. You can guess the amount of smoke that came out of the PLC.(Not to mention the amount of overtime to get the plant runnung again)


http://www.plctalk.net/qanda/attachment.php?attachmentid=8638&stc=1

 

[Jeev]

"Supply for control circuit". What exactly is that 220V going to?

 

[briancr]

The 220V is for the contactors, psu's etc

 

[bkottaras]

Hi All

Would like to here comments from the members about running high and low voltage through the same relay

See attached pdf

Had a situation where the relay faulted and the high voltage shorted through to the low voltage. You can guess the amount of smoke that came out of the PLC.(Not to mention the amount of overtime to get the plant runnung again)


http://www.plctalk.net/qanda/attachment.php?attachmentid=8638&stc=1

We use a similar setup though we do not switch anything through the contacts of the (Pilz) safety relay (ecxept additional coils which are 24VDC).
We do pick up another contactor through the contacts of the safety relay (24VDC coil) that does all the make/break for the high voltage through its contacts.
Safety modules have minimum ampacity of switching through their contacts, that's why you had a meltdown.

 

[Jeev]

We use a similar setup though we do not switch anything through the contacts of the (Pilz) safety relay (ecxept additional coils which are 24VDC).
We do pick up another contactor through the contacts of the safety relay (24VDC coil) that does all the make/break for the high voltage through its contacts.
Safety modules have minimum ampacity of switching through their contacts, that's why you had a meltdown.

That's exactly how I do it. I try to keep all my safety related voltages 24V DC and use the Safety Relay to switch Safety Contactors with 24V DC coils. No phases or high voltages are directly dealt with by the the Safety Relay.

 

[Tom Jenkins]

If the relay is rated for the voltage and current being applied to both sets of contacts this should work. If you want insurance, put 24 VDC through both sets of contacts and use one of the contacts to pull in a separate contactor or relay for the 220 VAC.

 

[briancr]

Thanks Guys


I have allready modified the wiring as suggested above to only have 24v going through the relay and to operate a seperate contactor for the 220v.

This machine was supplied OEM from Germany. We have a couple of machines from the same manufacturer with the same wiring. Management did not want me to change the wiring, so did it on the sly without them knowing. There is no way I am going to spend many many hours busting my butt trying to find all the proxies that have been toasted.

Many thanks again

 

[Jeev]

Here comes my OCD. What safety category is this machine meant to meet? Don't forget you may also need to design with redundant safety contactors (not every contactor can be used for safety). Not only that, but when designing I also interlock the reset signal with the aux contacts on the safety contactors. In the even of a welded contactor, you won't be able to reset the relay.

If you require some design ideas for different category systems I can post up some schematics.

~Jeev.

 

[BobB]

Omron have a relay - G2R series - where the 2 contact sets are isolated from each other physically and from the coil. AB sell the same relay - Omron manufacture them for AB.