480V, 3ph, Ungrounded, GROUND INDICATION LIGHTS - is this the only way to detect?

[bdn2004]

Working at a new facility that is using 480V 3-phase ungrounded distribution power to the MCC's. Not really seen this before.
So the new MCC we ordered has these ground indication lights.

One side of each light is wired to the ground. The other to the phase. So there is 480V across each of the three lights. If the phase it's connected to goes to ground this would cause both to be at the same potential and the light goes out. Right?

Is there anything new in technology to replace these? Is there any type of remote indication available for these that a burnout has occurred? Should there be warnings on the lights indicating they are not 120V?

Would appreciate anyone's experience with these kind of lights and known issues. Thanks!

 

[jraef]

It's a very old technology, but honestly, very simple, reliable and cheap. I was an electrician at a steel mill 40+ years ago, we used that method. When one light went out, all you knew was that one phase was going to ground, then we jumped into a golf cart and drove around looking for machinery that was not working to find the fault. Usually each light bulb had a small transformer on them so they are 120V (or in our case 240V) by the way. If they did it with pilot lights, most likely they are transformer type pilot lights with 6V bulbs, or LEDs now. In our case we had a simple test circuit hooked up to a 230V source that energized (while isolating) all 3 bulbs. So if one went out, you pressed the test button to see if all 3 lit up. If they did, you had a GF on the dark phase. If the one dark one did not light up in Test, the bulb was replaced. If none of them lit up, the GF was on one of the test circuit feeder phases, still confirming it was real.

Yes, you can spend a lot more money on a Ground Dectection Relay that basically tells you the same info. The main advantage is if you have a network like Ethernet/IP, the GDR can any use it across the network. Many Feeder Protection Relays offer this function, so if you are using one anyway, just make sure it has this.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[davidbeach]

If done right, under normal conditions all three (or often all 6 with pairs in series) are dim. When a phase is grounded one will go out and the other two will become bright. One out and two dim is a burnt out bulb.

 

[electricpete]

We have this setup at our plant on ungrounded system load centers. It works fine.
However it is not the only means of ground detection. ... each ungrounded system load center also has a ground detection relay fed from PT's for alarm.
The lights are only a redundant/diverse indication.

=====================================
(2B)+(2B)' ?

 

[bdn2004]

So they are special lights with a transformer built inside that makes them 120V, that's new info. The MCC manufacturer drawing doesn't show this transformer. If the load...ie the lights, only consume the power they draw, What would cause the other two lights to burn brighter when one of the phases is at ground? Or is this more hidden circuitry not shown in the simplified version?

 

[jraef]

The two light in series system is typically on a wall in the electric shop, not what you typically find in an MCC. They are likely just using transformer type pilot lights. If you don't spec anything in particular, you will get the cheapest thing they can get away with. Did you get details?


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[bdn2004]

No details that I can find. I guess this their standard offering. And there are no details required of the manufacturer per the client spec either...so I guess cheap is what we are going to get.as you know, you only what you ask for.

 

[davidbeach]

Will no ground fault there'd be 277V across each. With a ground fault there'd be 0 across one and 480 across the other two. If there's one burnt out lamp there'd be 240V across the remaining two. Get an LED indicator with a constant brightness driver and you lose that bright/dim information.

 

[waross]

The pilot lights will be 480 Volt units.
I have seen volt-meters used also.
The meters normally read 277 Volts. In the event of a ground fault one meter goes to zero volts and the other two meters go to 480 Volts.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[MatthewDB]

The push in the North American market is to go away from ungrounded systems. The preference is to switch to resistance ground. The danger in ungrounded systems is re-striking faults and transient over-voltages (System capacitance causes arcing 120 times a second at a small gap fault; each arc acts as a spark gap transmitter making a broad frequency range. This excites resonance resulting in up to a few kV to ground.) It is not uncommon for a single fault to create another one elsewhere in the system. A resistance grounded system is free of this risk.

In the IEC market the push is to go to insulation monitoring systems. They inject a low frequency monitoring current and measure the system impedance to ground. They report the resistance and capacitance to earth. They are setup to alarm once the resistance drops below a set value. Here is one brand: vigilohm - Schneider Electric that is UL listed. In the IEC market they also use surge limiters that convert the system from ungrounded to grounded on over-voltage. This protects in case the transformer faults. However, I haven't seen any that are UL listed so they are out for use in the US.

 

[bdn2004]

Attached is a copy of the wiring diagram for these ground indication lights.

 

[crshears]

Looks to me like Bender [ www.bender.org ] makes UL certified ground detectors for ungrounded systems...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[jraef]

So that looks like an Option -88BT for a Bulletin 2193M Main Breaker in an Allen Bradley CenterLine MCC. They are transformer type PPT pilot lights, meaning each light has a 480-6V transformer and uses a 6V lamp. You can get that as a 6VLED if you ask.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington