600V Autotransformer + Zig-Zag

[big john]

We have a lot of facilities with ungrounded 600V systems. We need a lot of 480 which we usually get from buck/boost transformers. The problem is, these systems are still ungrounded. These are hydroelectric stations, and I have very little faith in the ability of the ancient ungrounded systems to insulate well enough to keep someone from getting electrocuted in an environment that's all water and metal.

I'd like to add a zig-zag transformer to the to the 480 that we're deriving from our autotransformers. That would give me the ability to land equipment grounds and provide a measure of safety.

Does anyone see a problem with this?

Thanks a bunch.

-John

 

[benaround]

John,

Have you looked at 450.5 in the NEC ?

 

[LarryFine]

John, there's more than one way to look at the relative dangers of grounded and ungrounded systems. Grounding one conductor of a system locks the voltage-to-ground of each of the other conductors.

Yes, that does limit that voltage(s) to the system voltage(s), and minimizes the likelihood of higher voltages being imposed by, say, accidental contact with a primary conductor, or maybe even lightning.

However, the grounding also assures that the system voltage(s) are present, relative to earth and grounded surfaces, assuring the likelihood of shock or electrocution if contacted while grounded.

 

[big john]

I think I realised a problem last night: I don't think I can ground my derived 480 without also inadvertantly grounding my 600, because my 480 is from an autotransformer and not a seperately derived system. That will create all kinds of issues in itself.

Larry, I hear what you're saying, but given the age and condition of these systems, I would be extrordinarily surprised if there is not enough leakage/capacitive coupling to allow enough current to flow during a ground fault to allow electrocution. On a couple of the smaller systems, there's no ground-fault detection whatsoever. For all I know, they could be unintentionally grounded to begin with.

Bottom line, I don't believe they are safe from the standpoint of an electrocution-risk.

-John