# Ground Fault Protection in Unearthed HV Systems

I'm working somewhere that uses an unearthed 3.3kV network for safety critical drives. I've been learning about ground fault detection on ungrounded systems, using VT's and measuring voltage across a broken delta arrangement to detect ground fault:

(Source)

I'm confused as surely the star point on the VT's effectively ground's the ungrounded delta connections, negating the purpose of having an ungrounded system in the first place? Each phase will now have a reference to ground, allowing for phase-ground faults?

• Your correct in that something is wrong. The 4800/120vac transformer acts as a ground reference, so corner-grounding a delta feed would create a short circuit. You can do one or the other-but not both.
– user105652
Commented Aug 2, 2016 at 21:58
• I'm not familiar with high-voltage systems, but could you insert a large resistance between the line and the VT used for measurement? Commented Aug 2, 2016 at 23:19

It may seen like that but it is the only way to detect a phase to ground leak/fault in the 'isolated' system.

If you did not have that ground connection then no current would flow into the ground fault and you would not be able to detect it. With this circuit you will have no current in the earth point if you have a balanced load (and supply).

If you have a fully isolated system you cannot detect a faulty to ground from any phase (or neutral) as no current flows. All that happens is that your system gains a real ground reference at some point (perhaps not the best point) but no ground/fault current will flow until some point of your system connects to a point with a potential difference, this will cause a current to flow.

Having no way to detect the single fault is risky especially with high voltages or current. If your circuit must be floating it could float at any voltage and possibly exceed insulation capabilities.

On consideration I think it may be the best way to manage the 'isolation' and it will reference your circuit to ground at the virtual neutral point. If your circuit must be isolated so it can float to the load potential then you should reference the ground to a suitable neutral point there.