Abnormal Voltage Phase to Ground .

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sbarkah

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Our plant has voltage system 415V; 3Ph; 3W and located on offshore (Floating Platform). The Ground and Neutral is common.

At main busbar, I measured the voltage between Phase A and Ground = 5 V; Phase B and Ground = 415 V; Phase C and Ground = 415 V. The voltage reading should be 240 V.
One day before measuring, one of the feeders was hit by lightning ?
What we have to solve the problem?
Many thank.
 
Re: Abnormal Voltage Phase to Ground .

If this an ungrounded system, it sounds like a phase A to ground fault. Ungrounded systems should have ground fault indiators.The 415V figure sounds unusual.
 
Re: Abnormal Voltage Phase to Ground .

3P-3W does not have a neutral. You indicated a ungrounded system. I agree from the voltages you indicated phase A has a ground fault. I also agree you should have a ground fault indicator system telling you that.

Find your ground fault on phase A.
 
Re: Abnormal Voltage Phase to Ground .

It sounds like you have a ground on A phase. If you have a 3ph 3 wire you don't have a neutral. If you did it would have triped OCPD. You should have a ground fault indicator system. Most time this is three lights. The one for A phase should be dark and B & C should be illuminated. If you don't have a ground indicator system take your meter and check the voltage to ground as you turn off and on each breaker one at a time. When the voltage to ground goes to 240 when the breaker if off or all three of your indicator lights come on you have found the circuit with the ground fault. Then all you have to do it fix it.
 
Re: Abnormal Voltage Phase to Ground .

If in deed you have a 3 phase 4 wire (your statement does not make complete sense.) then your neutral to ground bond is OPEN and you have an A phase ground fault. If you have a 3 phase 3 wire then the fault is as noted in the other post.

Either way this should be addressed ASAP.
 
Re: Abnormal Voltage Phase to Ground .

Yes, absolutely that our system is 3Ph; 3W. Then to LV load via Transformer with connection Delta to Wye with NGR. Incoming LV switchgear has 3Ph; 3W.
Our grounding is floating because we're on offshore (Floating Platform unit), water depth is 3200 ft from sea level to sea bed.
Other 240V load (lighting, etc.) is supplied from phase to floating ground.
It looks like the Phase A is grounded then the ground is to become Phase A. That's why voltage between Phase B to Ground and Phase C and Ground is equal with phase to phase (415 V).
We are now waiting for plan shutdown for checking each load (MCC) which is grounded.
Many thank for your kindest assistance.

sbarkah
 
Re: Abnormal Voltage Phase to Ground .

sbarkah,
If you have a 3p 3w system you should not try tu check it to ground it is not designed to read to ground. It is designrd to read phase to phase. You could blow your voltage meter up checking phase to ground. Also we have alot of water wells that are wired with a grounded phase. This is to help prevent damage from lightning strikes.
Barry
 
Re: Abnormal Voltage Phase to Ground .

If you have a 3p 3w system you should not try tu check it to ground it is not designed to read to ground. It is designrd to read phase to phase. You could blow your voltage meter up checking phase to ground.
What type of meter do you use? At my plant, we use an 3p 3w ungrounded Delta system. The way we check for grounded phases, besides our "Ground Fault" indicator lights, is with our voltmeters which are fluke brand.
 
Re: Abnormal Voltage Phase to Ground .

bbeste & super dave,
I've just found the phase grounded caused by one of the load (Immersion Heater for Flash Gas Compressor). The fault finding by racking out one by one of the starters of MCC after our process was to be shutdown.
I checked the load (heater). It looks like the Phase A connection grounded to the junction box and thermostat was broken.
Now our system is back to normal. Phases to ground is 240 V.

BTW, I measured the voltage pahase to ground by using voltmeter (Multimeter) BLUEPOINT type EEDM504B.
Could I blow my voltage meter if checking phase to ground using the meter?
I always use this meter for continuity some fuses without disconnect those fuses from line (still in hot/live) in Voltmeter mode. Please advise if I made mistake to the multimeter.

sbarkah
 
Re: Abnormal Voltage Phase to Ground .

Originally posted by sbarkah:
Could I blow my voltage meter if checking phase to ground using the meter?
With a 3-wire delta system P-G readings only become important if you have a grounded phase conductor. Take your example; one read 0 while the others read 415. In normal operation, there should be enough capacitive coupling to ground to get a reading like you are seeing (240). In a small distribution system it may be higher or lower. As long as one is not zero, or near zero, it should be OK.
 
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