Current test switch burden

[Downstream]

Does anyone have any test data for the burden of a FT-1 current test switch? I am trying to do a burden calculation for a CT circuit with 2 relays and 3 FT-1 test switches in the circuit. I am going to use .1 ohm per test switch and that to include the termination contact resistance but don't want to be to low or to high.

Thanks

 

[dpc]

Sounds really high to me. One hundred feet of #10 AWG is about 0.1 ohms. If you have a Low Resistance Ohmmeter, you could put about 50 amps through one and measure it. I'd expect something less than 0.01 ohms. I've never run across anyone who included this in burden calcs, FWIW.

 

[cuky2000]

I am agree with DPC, the resistance value is negligible. For practical application test switches are ignored.

 

[DTR2011]

A competitor to ABB FT switch (Secucontrol.com) lists their FT compatible switch having a contact resistance less than or equal to 2 milli ohm.

 

[Downstream]

I found this article on the burden of test switches indicating the resistance/burden of the FT-1 test switches should be considered. See page 17 of the artictle. If this is true, I don't know why the manufacturers don't put in this information into the specs on their test switches.

It was by secure control
"Test Blocks, Plugs, and Probes – or When
the Dynamo Meets the Integrated Circuit"
David R. Blair
BHP Energy Solutions, Ltd.

http://www.google.com/url?q=http://...QQFjAI&usg=AFQjCNF2sgOV_uYF6nSn4NGav6HzuruVZw

 

[dpc]

I think the author is trying to sell something. Obviously there is some resistance. Smallgreek's reference of .002 ohms seems reasonable to me. Don't forget to add in the resistance of the crimp connectors, and adjust for high ambient temperature on the hottest day the installation will ever experience.

If neglecting this impedance was creating actual problems with CT saturation and burden error, I think the issue would be known and practicing engineers would be including it.

 

[scottf]

It's generally accepted practice to ignore the test switch resistance, as it's almost always dominated by the lead's resistance.

I formerly worked for a company that made test switches and did some informal testing that showed an average of around 1-2 milli-ohms per circuit (meaning 2 test switches) and this included all of the crimp connectors, etc...

 

[DRWeig]

Couple of mΩ matches my experience with measuring switches and contactors.

Good on ya,

Goober Dave

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