ground fault indication 1

[mcfluffie]

I want to stepup voltage from 120/208V system to 600V to operate milling machine and will require ground fault indicating lamps (3). Commercially available devices seem expensive, does anyone have a more reasonable method that is acceptable to inspection? Is simply 3 transformer type pilot lights good enough?

 

[peebee]

Sounds reasonable to me, but the only one who could really answer that would be your inspector.

 

[busbar]

The lamps may do the job. They need to be large enough to offset capacitive charging current in the 600V system. What is the anticipated transformer kVA rating?

{Aside — In NEC regions, if the transformer is 3-wire ∆-∆, the 208V overcurrent protection can also provide overcurrent protection of the 600V winding.}

 

[mcfluffie]

Thanks peebee & busbar. The trans will be 30 KVA

 

[busbar]

At 30kVA, the ungrounded-system charging current will be fairly low—ROUGHLY 30mA. [One amp/MVA rule of thumb for low-voltage ungrounded systems.]

For novelty, if the powered equipment would work OK at nominally 624 volts, one could conceivably use 1ø transformers with 600V-rated insulation {most UL506 units seem to be.}

Three 240:120V transformers could be configured in a wye-autotransformer arrangement for ~360V ø-n. For three 1ø transformers, all X2s connect to 208V neutral—each X1 connects to a 208V phase and respective transformer H1—with 624V 3ø on the three H2 leads. The “600V” neutral would be solidly grounded by default through the 208Y/120V 4-wire system. Taps may could be used for voltage adjustment. Smaller units with 240 x 480 Volts – 120/240 Volts • 2 – 2.5% FCAN + 2 – 2.5% FCBN Taps are catalog items.

 

[mcfluffie]

Interesting idea busbar,although I already have a 3 Ph.,
30 Kva,600-120/208 V transformer.Are you suggesting that this alternate method would not require the use of indicating lamps due to the "600 V neutral" connection?

 

[Shortstub]

Presuming that there is a control circuit, then it should not operate Ungrounded. If supplied by transformer, then neutral must be grounded. And, if grounded, then, ground-fault lamps are useless.

 

[peebee]

I disagree with you, Shortstub. I do think grounding is a good idea, but it's not strictly required by NEC. See NEC 250.20(B).

 

[busbar]

mcfluffie, if the 600V system is solidly grounded, as would be the case with an autotransformer, then a ground fault would rightly operate an overcurrent device on “first fault,” and the lamps would be of no use as mentioned by Shortstub.

If the subject transformer is 600∆-208Y, in a stepup mode leave the XO connection floating.