Why is a 7.5 Watt light bulb used in series w/fluke when doing in service readings on POTS 1

[BSEEDan23]

A 138KV CCVT was replaced and the CCVT was to be energized with no load while in service checks were done.

Basically the procedures call to use a 7.5W light bulb in series with the fluke. One lead to the neutral test switch and the other lead from the fluke to the light bulb to the A phase test switch. Vac measurements are taken from A-N,B-N,C-N, A-B,A-C,B-C and then each phase is compared with the each phase on another in service bus. One lead of the fluke to A phase Bus 1, the other lead of the fluke to a light bulb and the light bulb to A phase Bus 2. Negligible voltage, A phase Bus 1 to B Phase bus 2 gives phase to phase voltage


I understand the light bulb is used for protection. I am trying to understand how. I have been told that it does NOT draw enough current to cause equipment to trip out in case the lead was placed on the wrong test switch. Any documentation that can clear this confusion up for me? I see the light bulb used a lot when lamping Current transformer circuits. This is something I will be doing eventually and want to understand the concept.

 

[Marmite]

It's probably a precaution against someone trying to measure AC volts with the Fluke accidentally set to AC Amps.
Some Companies mandate the use of fused test leads for voltage measurement. In one of my former employers it was a disciplinary offence not to do so.

Regards
Marmite

 

[ScottyUK]

Using lamps in a CT circuit sounds distinctly risky given how fragile filaments tend to be once they've been warmed up a few times. If the filament fails when the lamp is in series with a CT then things could get quite exciting. Are you sure about what you are seeing?

 

[BSEEDan23]

Marmite, I believe you are correct. It's part of our requirements. I'm trying to understand what would happen if the light bulb was not in series with the fluke and one lead of the fluke was attached to in service A phase POT test switch while the other lead somehow fell and connected to ground?

Procedures:
When using live potential feeds for reference on a phase angle meter or voltmeter, use a low wattage bulb in series between the source and meter.

ScottyUK,
Current Circuit Lamping
Lamping of current transformer secondary circuits is a method used to perform a final point-to-point check to verify all equipment connections. Details of the test may vary, but any method used must prove the following:

Test Equipment
AC current will be supplied by a lamping bulb, a light bulb connected to a grounded source. Magnitude of current will be measured by a standard ac ammeter with appropriate test plugs. Direction of current will be determined with a voltmeter or multimeter.

Lamping Method
Verify that only one ground is present on the CT circuit. Find and lift the removable ground link. Connect the lamping bulb to all accessible circuit connections on all phases and the neutral. Most often this will be the first terminal block from the CT. The lamping bulb should not light.

 

[ScottyUK]

Old-school but I can imagine that it works OK. Thanks for the reply.