overload relays

i have recently came across a situation that a contractor was removing the wiring from the N/C contacts on a overload relay to the motor starter. He was then rewiring the contacts on the overload relay to provide a PLC an input, which then was used to shutdown the output. I am concerned with the use of a PLC to provide the shutdown of the motor in the case of an overload. Does anyone have any opions on this?

 

[coolfish]

I would say that good practice is to hardwire the N/C contact in series with coil. If your O/l has an available NO contact, you can still connect this to a PLC input. You can then also trip the motor via logic in addition to the hardwire NC contact.

 

[peterb]

Good practice keeps protective functions separate from PLC logic, unless great care is taken to provide a true failsafe system.
If your system needs to monitor operation of the overload and there is no second contact available, then wire a PLC input from the hot wire to the coil side of the overload contact. The input will be normally energized, de-energized only when the overload N/C contact opens or the starter supply is lost.

 

[buzzp]

The more 'devices' that the signal has to go through the higher the chances are for failure to shut the motor off on an overload condition. So if you hook the signal to the PLC then if the overload fails or the PLC fails, then the motor won't get shut off unless it is designed to be 'fail safe'. This terminology, from OL standpoint, means if the OL is sitting with no power then there is an open contact, usually labeled NO and C. When power is applied, this contact closes until an overload occurs and then it opens the contact. Works basically the same except if internal circuitry fails, then it is likely the OL will not be able to keep the relay energized (power supply circuit failure, microcontroller failure, etc..). So if an internal problem occurs, the motor will be shut off, letting maintainence know there is a problem. If a NC contact is used and an internal failure occurs, you will never know it until the motor burns up or fuse blows, etc. I would use a fail safe design and would not be afraid to allow the PLC to shut the motor off through the use of contactor/relay. If both outputs are using the NC contacts then this would not be okay in my book. If the OL was NC and you used the NO on the PLC then the PLC is fail safe but the OL is not. This will reduce the chance of allowing the motor to run if the PLC fails. Hope this helps, Buzzp. NOTE: I would use a solid state overload relay instead of a thermal. Thermals are usually not fail safe and solid state overloads vary by mfg or they may provide SPDT contacts.