Slave Relays

[Luke Barber]

I am going to install a Siemens PLC on a project. It has relay outputs and I am in 2 minds whether I should slave the outputs through relays before driving contactors like ABB's A-lines or just directly connect. I've heard if the output from the PLC is going to a "high load" device like a motor operated valve or a motor starter I should always use a interposing relay no matter what kind of output I have on the PLC. Little things like pilot lights and outputs for permissive to other devices (voltage sense inputs) it would be OK wiring direct. Is that accurate?

Your thoughts would be appreciated.

[dekor]

Hi Luke,

that is a general rule to use small loads for PLC output, not only for Siemens.

The contactors usually generate "self induction voltage" and usually generate spikes between relay contacts that enable them ( on your situation PLC outputs). Those spikes will damage the relays of your PLC .

Think also for situation when you need to replace a cheap intermediate relay instead of an expensive PLC output module.

 It will be also more easy for debug.

My personal opinion is to use an "intermediate relay" even if the price will be higher.