Ground (fault) detection/detectors for ungrounded systems

[thunder15j]

We service several older electrical systems that are 480 volt 3 phase systems, some 240 volt systems, that are fed from our Poco by 3 wire ungrounded systems. (I've mentioned this in other posts) Our customer gear is rated 3 phase-3 wire. 100 and 200 amp meter sockets are 5 jaw. The systems are typically 480 volt Delta. Bear in mind that these systems are typically 25+ years old As per NEC 250.21 ground detection is required on these systems today but probably not when they were installed.

In the pursuit of safety on the 480 volt systems, we have installed a pilot lit ground fault detection system which consists of three 480 volt pilot lights connected L1-ground, L2-ground, L3-ground. On normal circumstances they will be dimly lit. Also a PTT(push to test) connection is made across 480 volts to prove the lamps will burn brightly when a fault would occur. In the event of a detected ground fault, one of the three lamps will go out and the other two will burn brightly. The equipment wiil keep running but the operator is now aware that a fault exists and must be promtly repaired. This is a configuration that I got off the MSHA site. It seems to work quite well.

I'm wondering what other ground detector systems are out there.

Also would this work on an ungrounded wye configured transformer that the Poco has provided? Customer gear is also 3 wire-3 phase.
Funny thing on this installation: I was getting ready to install the 3 light detection system and noticed I was getting 277 to ground on all three legs. Poco says that the neutral bushing on their transformer is not connected to ground and they have only three wire coming into the service gear. Poco says something about an induced voltage (cannot use the actual word on this site; b------ voltage ). Voltage was measured with Fluke 117 in low impedance mode.

 

[augie47]

Most of the detectors I see are just as you describe although some have more bells and whistles (literally).
I don't know that I have ever seen 277 to ground on all 3 phases of an ungrounded system. Was there any load or was this measured to a 0 load or open main ?
I've seen all manner of voltages but never anything that consistent.

 

[thunder15j]

The reading was taken with a light load on the system. Lights, receptacles that are fed from a customer owned transormer (480 x 120/240). Plant was down. Line to metal conduit ground.

Because of heavy rain and the main switchgear being located outside in the weather, I did not want to turn off the main and/or other feeder breakers to isolate the source of the voltage. I'll do that on another visit.

 

[templdl]

Why in the world would anybody want an ungrounded system unless it was to supply power to an industrial facility where you didn't want to shut it down because of a ground fault.
If you have a manufacturing facility where shutting down the process automatically would cost an enormous amount of machine damage and material loss ungrounded deltas are often used. As such a ground detection system is used such as the one described using lights to indicate the presents of a line to ground. BUT, what happens after that is the issue. The facility should be staffed by a qualified person who is knowledgeable about what procedures would be used to resolve the issue. If the facility is not able to react then there is not purpose to the ground fault indicator lights.

 

[rcwilson]

A more expensive but much safer and higher reliability solution is to switch the systems to high resistance grounded. Add zig-zag or wye-delta grounding transformers and resistors at each service. When a fault occurs, the system alarms and limits the fault to 5 - 10 Amps. The electrician still has to chase down and find the ground fault before the next one occurs. But the fault tracing feature of the grounding system makes the fault locating much easier and it can be done without having to de-energize loads.

In the locate mode, the grounding system shorts out a portion of the resistor to pulse the fault current a few amps about once a second. Clamp on a circuit or a conduit that is carrying the fault current and the pulsing current is easily seen. You can trace it to the fault location quickly. (If the fault is still there, if that load is still running, if you can get the clamp around the conduit, etc. the usual troubleshooting issues.)

For high reliability systems, a ground fault sensing donut CT is put in every feeder breaker and the signal sent to control modules that communicate with each other and a master control. When a fault occurs, the system alarms and identifies the feeder with the fault. If a second fault occurs the system looks at the pre-programmed priority chart and trips the load with the lowest priority. (Do a search for I-Gard for more information.)

 

[templdl]

A more expensive but much safer and higher reliability solution is to switch the systems to high resistance grounded. Add zig-zag or wye-delta grounding transformers and resistors at each service. When a fault occurs, the system alarms and limits the fault to 5 - 10 Amps. The electrician still has to chase down and find the ground fault before the next one occurs. But the fault tracing feature of the grounding system makes the fault locating much easier and it can be done without having to de-energize loads.

In the locate mode, the grounding system shorts out a portion of the resistor to pulse the fault current a few amps about once a second. Clamp on a circuit or a conduit that is carrying the fault current and the pulsing current is easily seen. You can trace it to the fault location quickly. (If the fault is still there, if that load is still running, if you can get the clamp around the conduit, etc. the usual troubleshooting issues.)

For high reliability systems, a ground fault sensing donut CT is put in every feeder breaker and the signal sent to control modules that communicate with each other and a master control. When a fault occurs, the system alarms and identifies the feeder with the fault. If a second fault occurs the system looks at the pre-programmed priority chart and trips the load with the lowest priority. (Do a search for I-Gard for more information.)

Yes, if the intent is not to corner ground or solidly ground the system as describle for some practical reason please consider this recommendation at the very least. It may not be cheap but the system as you have described it may not be cheap either after you have a failure.

 

[beanland]

Zero-sequence

Zero-sequence

The only other kind of ground detector I have seen is one that uses three 277V-120V transformers. Connect the transformers grounded wye on the 480V side and delta on the 120V side but leave the corner of the delta open and install a light. A ground on any one phase will cause the light to glow.

The 277V on an ungrounded system is from the capacitance in the transformer, it will provide enough leakage current to give you 277V to ground on each phase. In fact, a 480V delta transformer may do the same because of the leakage capacitance in the transformer. The same light system will work but only if there is some load.