What is the preference for people on FT switches?
Do you wire it such that the VTs are connected to the “Blade” side of the switch and the relays are connected to the stationary side, or Vice-Versa?
In one aspect - it makes sense to have the blade side connected to the VTs as that is consistent with the standard wiring diagram for FT switches and works better from the back of the switch (odd terminal But when wired the other way that the blades go to the relay, then there is reduced shock hazard at the blades are not live.
Thoughts/Comments?
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Typically, relay to the top, VT’s to the blade. Seems pretty standard in the Washington DC Area
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I’ve seen both, but more often than not the blade side is to the relay to prevent live voltages from hanging out when they are pulled. Which makes the most sense to me. In my opinion, there doesn’t need to be a “standard” for wiring the relay to the top side for voltage circuits, as long as it is properly represented on the drawings of which side is which.
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Since Westinghouse made it’s very first draw out relays (1940’s), the standard has been that the blades go to the system and the jaws go to the relays.
Failing to wire this way renders the factory designed test paddle useless and can create even greater safety hazards.
The test switches are normally inaccessible due to the cover or relay case. If someone isn’t qualified to be around 120/69 Vac or 125 VDC, they have no business opening the cover in the first place. The AC voltages from the PT’s are normally fused for a few amps. There is no arc flash hazard and barely a shock hazard.
Every time I encounter a FT wired backward, I make a point of calling the designer / engineer and ask what their thought process was. Normally are following a standard that would apply to a power switch and have no concept of the other risks they are introducing.
The ABB FT manual clearly shows the system on the blades and the relays at the jaws. Failure to follow manufacturers recommendations comes with some liability.
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