There are (20) 400 watt MV High bay lights wired to a 30 amp 3-pole CB. There is a ground fault on the ckt. that is causing the CB to trip, shutting down all 20 fixtures. Someone repaired it once, but the same thing happened again. Although a neutral conductor was run to the 400-amp service MDP, it is just taped off and laying in the gutter. Voltage to ground is 277, so it must be a 480/277 wye service. The problem is that since ther are 20, and not 21 lights(divisable by 3) the load must be imbalanced. I think that the imbalance might be causing a higher voltage on some fixtures, causing the ballast to short. I'm thinking about simply adding another fixture, and replacing the existing fixtures with fixtures that have fuses. Any thoughts? Thanks.
[This message has been edited by Redsy (edited 02-19-2005).]
If the service is 277/480 volts then the neutral must be run and connected to the MDP. You have no return path for a fault. When you say "there is a ground fault on the circuit" do you mean a GFI breaker? ARe the lts connected 277 volts or 480? If its 277 volts that may be why the breaker is tripping.
#48766 - 02/19/0506:12 PMRe: Unbalanced 480 volt lighting ckt
Bob, The lights are connected at 480 volts off a 3-pole CB. Thay are connected 7 lights between phase A&B, 7 between B&C, and 6 bewteen A&C. When I say ground fault, I mean a short circuit to ground. The load conductors off the CB are showing less than 1 ohm to the conduit. I know that the neutral SHOULD be connected to the MDP, but it is not. It's a fairly old service, and it is connected at the service transformer. The conduits are providing the fault path. Although it may not have the lowest impedance possible, it's enough to trip the CB. I thought about trying to obtain a neutral buss, but I don't know if I'll be able to fit it in. Also, the I would have to replace all the fixtures with 277 volt units, obtain 3 single pole CBs, and pull a branch circuit neutral. I don't think the owner wants to go through with that expense.
#48767 - 02/19/0506:27 PMRe: Unbalanced 480 volt lighting ckt
Redsy, why would you have to replace the fixtures if you hook up the neutral conductor to a neutral buss. The fixtures are okay wired as 480 on a 3 pole breaker. The problem with this type of setup, you have experienced. One bad ballast will trip the breaker and you lose all the lights. Installing fuses for each fixture like you posted will help with that. I also do not see the need to install a seventh fixture on A&C. Al
#48768 - 02/19/0509:31 PMRe: Unbalanced 480 volt lighting ckt
Hi, Rojosy The voltage situation you mention would occur with 277 volt circuits that had lost the neutral, but 480 lighting should not be affected in this way. There's no need for an additional fixture. Replacing them with fused fixtures is a very good idea.
#48769 - 02/19/0509:45 PMRe: Unbalanced 480 volt lighting ckt
Redsy As has been said, there is no need to change the fixtures. Leave them connected as they are. I see no need for the GFI breaker. Why not change to a std breaker. The should solve the problem. Not a bad idea to install a fuse for each light.
#48770 - 02/19/0509:46 PMRe: Unbalanced 480 volt lighting ckt
Thanks, guys. electure-- how you been? I have been so busy, I only have time to stop by here when I need help ;D Anyway, It appears to me that the way the fixtures are connected, it is essentially a 3-phase delta load, much like a 3-phase unit heater. (If you draw it out, you'll see what I mean.) I see now that the unconnected neutral won't affect the 3-pole 480 volt (line to line) lighting circuit. I was confused at first. Thanks for the help.
The fixtures would need to be replaced because they are straight 480 volt ballasts and not adaptable to 277. But they will probably need to be replaced anyway, because I can't retro-fit (2) 480 volt fuses.
I'm not sure where the GFCI idea originated. I have a standard 3-ploe CB, and at 480 volts, each fixture would need 2 fuses.
[This message has been edited by Redsy (edited 02-20-2005).]