480V Ungrounded Delta System

[350MCM]

If one line conductor is grounded anywhere, that conductor will measure as grounded everywhere.

It does, just on that side of the switch gear line up, which only feeds a couple of bus sections. It’s a

If the A phase of the bad buss is disconnected (fuse?) and grounded somewhere out on the buss, then past that point, it would read 0 volts to ground. That still doesn't explain the 277 readings elsewhere.

I to wonder if the original readings are really what the OP was told or if they were confused.

yes they were looking at some bus that is fed from the the other side of the switch gear it’s a main, tie, main setup, with the tie out. But the line ups are identical. That’s were they were reading the 270ish (Iol) to ground. The other side of the tie has the ground somewhere. It does not follow past the tie breaker.

 

[LarryFine]

If the A phase of the bad buss is disconnected (fuse?) and grounded somewhere out on the buss, then past that point, it would read 0 volts to ground.

I must have missed the open fuse part; you're suggested the loads are running with two lines and a fault?

That still doesn't explain the 277 readings elsewhere.

With a typical voltmeter, capacitance would.

 

[zbang]

I must have missed the open fuse part; you're suggested the loads are running with two lines and a fault?

It's called a WAG, that's pretty much what we're all doing. (Are the loads running? If they're not, it could produce the symptoms.)

At this point, perhaps a one-line with voltages indicated would be really helpful.

 

[don_resqcapt19]

All the bus work is fed from one 13.2 to 480 transformer, with separate 1600A feeder breakers for each bus way. Common switchgear line up.

Then it is not possible to have just one feeder with a grounded phase. Either all of the feeders have a grounded phase or none of them do.

 

[kwired]

Then it is not possible to have just one feeder with a grounded phase. Either all of the feeders have a grounded phase or none of them do.

agree. Has to be more than one system there to have both a grounded phase and ~277 on all three phases to ground.

Is possible to see ~277 to ground on ungrounded if there is anything wye connected and some sort of fault/ bonding of said wye point.

Some misconnection in a wye wound motor or transformer could do it, yet still function "normally".

 

[synchro]

No sorry, it’s an ungrounded 3 phase system, with one leg grounded due to a fault. All the bus ways have grounding lights. But this section the ground indicators are not working, they are in the process of replacing.

Were the measurements on both buses made before these ground indicators were removed / replaced?
If not, and the bus with 0V to ground on one phase was measured before removing the indicators but the other bus was measured after removal, then perhaps one of the indicators was causing this ground fault and it went away when removed.

 

[paulengr]

So far the description of ungrounded deltas is accurate, but one more point to be aware of: they eat motors. A solid connection to ground does what is described but at that point all of the system capacitance...the voltage developed across the insulation between the phase conductors and ground...is draining through the unintentional ground. On 480 V it rarely exceeds 1-2 A. On higher voltage systems it is much more severe. But if it is a LOOSE connection, as most faults are, you get an arcing fault. Again this sounds relatively harmless. But what happens is that the repeated charging/discharging creates tremendous voltages to ground temporarily until they clear themselves by arcing through the weakest insulation in your system, every motor.

https://www.eaton.com/content/dam/eaton/markets/healthcare/knowledge-center/white-paper/transient-overvoltages-on-ungrounded-systems-from-intermittent-ground-faults.pdf

The second problem already hinted at is how to find the fault which as stated isn’t easy. It involves going around shutting things off until you find it.

The alternative is a high resistance ground. This uses a resistor between the transformer neutral and ground rather than a conventional solid bond. We want to limit current to a small level but at least 3 times the charging current so that we don’t just have a sort-of ungrounded system. 15 A is a good number for 480 V so the resistor is 277 / 15 A or 18.47 ohms. Such a resistor is about 18” long and about 8” square. Then apply your phase lights or more conveniently trigger an alarm if the current on the resistor goes over say 40% of that or 6 A or do it based on voltage across the resistor to capture resistor failures or ground faults (alarm if voltage exceeds 110 V). If you don’t have a wye, no problem. You can use a zig zag transformer or a wye-delta to create the missing neutral.

This is the best of both worlds. 15 A isn’t enough to even immediately destroy a #14 wire so damage is minimal. The system capacitance is continuously drained so transients never exceed 480 V. It can still operate indefinitely with a fault. But now you can use ground fault relays as well as simply measure the ground fault current (put your meter around all three phases) and follow it to the source without shutting anything down.

You still can’t have line to neutral loads but it eliminates all the disadvantages of both a solidly grounded and ungrounded system with a simple, safe, easy to maintain alternative.

Photo attached of a custom made high resistance ground stuffed into an existing fused disconnect.