FT Test switch wiring scheme - relay output contacts 3

[mvsubstation]

Just Curious anyone's opinion in regard to FT switches for Relay output contacts for close permissives and trip circuits for breakers. I have noticed some Switchgear manufacturers provide an FT switch on both sides of the relay output contact, and other manufacturers provide an FT switch on only the side opposite the breaker coils. Any opinions on which method is better? Seems like the only reason to provide a switch on both sides is to ensure that there is no voltage present on both of the terminals of the output contact (the circuits would be powered by an ungrounded 48VDC source in this case).

 

[jghrist]

I've only used one set, on the side of the breaker coil. In my opinion, this adequately serves the function of preventing breaker tripping when testing the relay. I see no problem with voltage being present on one side of the output contact.

 

[davidbeach]

If all you want to do is to block the circuit then one blade is sufficient. But if you want to intercept the circuit and run it to a breaker simulator during testing then two blades will be far more convenient.

 

[DTR2011]

"powered by an ungrounded 48VDC"

This would be a good reason to have both sides of the relay trip contacts available. In systems that have a battery ground detection circuit, DC voltage measurements are often made with reference to ground.

Others may cite safety when testing the relay (testing on dry contacts).

In more complex schemes, where DC control voltage may come from several sources, it is a convenient means of having a place to prove isolation.

If there is a limited amount of panel space, or a simple protection scheme, one could argue a single cut out is sufficient. If this is a new installation and space is not an issue, having both sides available makes like easier for the relay / commissioning folks, especially when modifications are made in the future.

 

[mvsubstation]

Thanks Guys; very helpful input!