Detect Local Disconnects

Compass · Jun 1, 2015

Hi

My boss wants the plc to detect whether a local disconnect is open or closed on many different 600 VAC panels. Ideally there would be contacts on the circuit breaker or the mechanical switch, but it's rarely that simple.

Ofcourse I could buy something to add on to the panels, but are there any other sugestions?

Bit_Bucket_07 · Jun 1, 2015

Most disconnect switches support early break auxiliary contacts for this purpose. An alternative would be to install a 600 VAC coil relay to detect the presence of voltage.

JesperMP · Jun 1, 2015

Do you want to detect exactly that a service disconnect is open ?
If so, there is no othe way than having an aux contact on the switch, and the signal has to go all the way back to the PLC.

Or do you just want to detect that "something is not alright" ?
If so, it may be enough to sense if current is flowing. You can do this with just one phase, or with all phases.
We use simple inexpensive current switches for this purpose. Since they sit in the main control panel, you dont have the hassle with getting the signal all the way back from somewhere in the plant.
This is one of the types that we use: Carlo Gavazzi DIA 53S72420A.
Just let the motor lead through the hole in the middle (loop several times for small motors) and send signal the output to a PLC input.

osmanmom · Jun 1, 2015

what the consequence if get a false signal by using this aux contact/relay,

mbartoli · Jun 1, 2015

When I worked for an OEM equipment supplier in Detroit, we specifed auxiliary contacts in each critical disconnect switch that would indicate if the disconnect was closed, connected as a DI to the PLC. If this is an existing system, and the disconnects do not already have the contacts, it should be fairly easy (and relatively inexpensive) to add them. Wiring back to the PLC could turn out to be more involved (read $$), but this would be the most reliable way, IMHO.

Compass · Jun 1, 2015

JesperMP

I think my boss wants exact detection, but I know we settle for less underground, so he might change his mind.

MBartoli

I don't have all the numbers on hand but some of the switches are:

Allen Bradley 140U-K3X3Moeller N7M6-100 Contacts 15-16 are NHI, This can be used in the PLCMoeller N7M9-250

silva.foxx · Jun 1, 2015

Power a relay via supply voltage and switch a signal into the PLC... or is that too simple?

Doug-P · Jun 1, 2015

It'll be a pain doing this but there is an upside - you can include the disconnect status in the logic and see right away if some distant device isn't running because somebody has a panel open or locked out.

jstolaruk · Jun 1, 2015

Aux contact on the disconnect is standard practice in big 3 automotive, including the main (PLC and HMI powered off a lighting disconnect). On rare occasion the 3 phase power is monitored.

Aabeck · Jun 1, 2015

If you monitor the 3Ø power it will show any power problem while the disconnect is still closed (usually single-phasing) If you want to ensure there is no power going through then you would need 3 relays, either one for each phase, or one lead to ground & make sure all 3 are off - if 1 or 2 are off you have a power or fuse problem.

mbartoli · Jun 1, 2015

jstolaruk said:
Aux contact on the disconnect is standard practice in big 3 automotive, including the main (PLC and HMI powered off a lighting disconnect). On rare occasion the 3 phase power is monitored.

Exactly my point...

Racer5039 · Jun 1, 2015

Check out RIB relays. Lets you interface with a lot of things in many ways
http://www.functionaldevices.com/building-automation/display.php?model=RIBH1C

JesperMP · Jun 2, 2015

silva.foxx said:
Power a relay via supply voltage and switch a signal into the PLC... or is that too simple?

You mean to power a relay on the output side of the disconnect.
You get false alarms when the supply voltage is off. So it is a fall-back solution only if a simple aux contact is not possible.

milmat1 · Jun 2, 2015

AUX Contacts on the disconnect/breaker.......

Simple effective and cheap...

Detect Local Disconnects

Compass

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Bit_Bucket_07

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JesperMP

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osmanmom

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mbartoli

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Compass

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silva.foxx

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Doug-P

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jstolaruk

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Aabeck

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mbartoli

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JesperMP

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milmat1

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