AC Control Relays and Wire Lengths 1

[cabletray]

I remember an EC&M article back in the '80's about problems with AC control relays having long wire connections. The article included tables based on wire size, and probably voltage, and provided recommended maximum wire lengths for use with AC relays to avoid disruptive levels of capacitive charging current between two wires that would interfere with relay operation. The Rotork Electric Valve Actuator catalog from that era also discussed the problem, and recommended using DC control relays in their actuators for long control wiring runs.

I've never seen this discussed anywhere else. Can anyone describe the problem in detail and recommend tables based on testing different wire sizes and types?

 

[resqcapt19]

take a look at this document from Square D

http://www.squared.com/us/products/...29214c232a6a852568390055a15d/$FILE/M-379F.pdf

Don

 

[dpc]

The Square D paper should give you the information you need. It is a real problem. The capacitive reactance in the control wiring can pass enough current to keep the coil from dropping out even when the remote contact is opened. For really long runs, you can use interposing relays with dc coils.

 

[itsmoked]

Gezz I never heard of this before... But that paper is a good one! Thanks resqcapt19.

 

[Hewlett]

Good article! experienced this problem once with an emergency stop being released "repeatedly and mysteriously". There was 3-4 emcy stop buttons with NO contacts several houndred meters from the control cabinet, making a good capacitor...solution in that case was selecting a considerably bigger relay...

 

[Skogsgurra]

Emergency stop with NO contacts? Are you serious?

Gunnar Englund

http://www.gke.org

 

[itsmoked]

Hewlett; They should all be NC and in series!

 

[Hewlett]

I am, and I hope my memory serves me right when I say this circuit with NO contacts was an "additional" order to control system, via control relays and PLC in control system.....as I remember, each button also had a NC contact that would open the main breakers...