Making Floating High Voltage AC Measurements With Vintage VOMs

[KJ-90]

I was wondering if it is safe to measure high AC voltages ( e.g. 4000 VAC ) from an energy limited source ( e.g. a 4000 V no CT plate transformer, max output 50mA ) using a Simpson 260 series 3 having the 5000 VAC input terminal, and neither side of the circuit is grounded. The meter's common terminal is not connected to ground either. The manual for the Simpson 260 series 3 gives no advice on this, nor the maximum common mode voltage with respect to ground. As we know in this measurement, the meter is floating. Each transformer lead has 2000 VAC on it. One lead of the transformer goes to the meter's 5000 VAC input, through the usual voltage attenuator network, then is at a low potential for the meter rectifier. However, the transformer's other lead goes to the common terminal, which doesn't have an attenuator stage similar to the other input. Wouldn't this kind of measurement damage the VOM, and if so, how? If not, why not? If damage would result, how are special meters that can make this measurement type designed with respect to the common terminal? The later Simpson 260s give the maximum common mode voltage as 1000 V either AC or DC, but their circuits haven't changed all that much, except that the 5000 V AC\DC jacks are no longer present.

By the way, this combination of voltage and current is highly dangerous, so all electrical safety precautions must be taken, which I am thoroughly familiar with.

[alm]

The manual makes no mention of a maximum voltage relative to ground for the common terminal, so it should not matter how you connect the leads (think a bird on a high voltage wire with a large value resistor on a long wire reaching down to ground and a small value resistor between the bird and the high voltage wire). As long as insulation breakdown to ground is not possible, the meter should not be damaged. However, much of the meter will be at a potential near the common terminal, so I think it would be safer to ground the common terminal, if this is feasible with your device under test.

Obviously the Simpson does not meet modern safety standards and it should be completely hands off (even the 1955 (series 2?) manual says so) and ideally behind plexiglass. Keep in mind that the 5 kV feature was removed decades ago, so even by then it was apparently not considered safe.

[Ian.M]

Stand the meter on an insulator you are certain is good for 10KV, and be *VERY* careful how the leads run, or better yet, make up a temporary set of leads from >10KV rated EHT wire, with plugs to fit the meter and solder the other end to the D.U.T.   Working 'hands free' is *ESSENTIAL* - don't touch anything till the power is off and all large caps discharged.

[schmitt trigger]

Fully and completely "Hands-off" while HV is present would be also my preferred recommendation.

[Vtile]

I have measured 50kV dc on these old meters with diy voltage divider 250 of megaohm impedance (or were it "only" 200 meg can't recal) made of 25 10meg HV resistors hanging from the roof . Input were a bit under 3kV on the old Taylor meter. The sound were interesting. Beware.

Your measurement sounds as dodgy as my 50kV. Think it twice and get something to discharge all surfaces after you turn the power off and do not touch any items in circuit including the table and other structures that hold the circuit. also figure out before hand shielding with ground wiring for creepage currents etc.