High resistance ground system

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windingelectric

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Hello, this is my first time posting a topic so I hope this is in the correct spot. I was called in to a facility to look at 2 hoist drives on cranes that went out. Upon metering the incoming power I found that b phase was grounded. I went to maintance and told them what I found. We went to the main gear room of the plant and the ground resistor was extremely hot. I told them that it needed fixed before we blew up 30,000 in new drives.

The service is a 4,000 amp 480 volt 3 phase 3 wire system. On their 1 line diagram it shows the 25kv feed from the utility through metering service disconnect then feeding the transformers. It shows a delta primary and a wye secondary but only 3 wire system. I am not very familiar with this type of distrubution. Maintance did turn off as many loads as they could but could not clear the fault.

Also the ground indicator lights have been unhooked from the system, I hate to call osha but they not wanting to fix it means that I can not fix the cranes and over the 4 hours I was there the fault got worse. It want from 20 volts to ground to 35 volts to ground.

Can anybody recommend a good reference to learn more about this kind of system. Also would you notify osha or somebody else?

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Hey there Wingding, did you test all phases to ground from utilities? How about from the main breaker to transformer, etc, to the distribution? Did you mean ground conductor was hot? It seems like you may have isolated a problem with B phase


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He means that the high value neutral to ground resistor in the otherwise ungrounded system is getting hot.
In an ungrounded system no fault current flows at the first fault but the second fault may be catastrophic in terms of current.
A low resistance ground will limit the first fault current but load the resistor so heavily that OCPD must operate to protect it.
A high resistance ground bond on the other hand can be left connected during a fault but the current in the resistor must set off an alarm that must be acknowledged


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If they won't let you fix it then there is not much you can do.

It sounds like what you have is a resistance grounded WYE system with a fault on the B leg. These systems are designed to allow operations to continue after the first fault to ground in industrial sites where loss of power would be a safety issue, but still require the fault to be fixed once it is identified. Eventually there will be a second fault to ground and that will shut down their operation, and at that point they should be more than happy to have you fix it. I heard that the second ground fault usually occurs within 2 weeks of the first so you should not have long to wait.
 
windingelectric said:
Also the ground indicator lights have been unhooked from the system...
Click to expand...

Ground indicating lights are not required on high-resistance grounded systems.

480V Systems can be solidly grounded, high resistant grounded, or ungrounded. Simply knowing the connection of the supply transformer windings is not sufficient for identifying the system. You need a 'one-line' of your grounding system.
 
windingelectric said:
Hello, this is my first time posting a topic so I hope this is in the correct spot. I was called in to a facility to look at 2 hoist drives on cranes that went out. Upon metering the incoming power I found that b phase was grounded. I went to maintance and told them what I found. We went to the main gear room of the plant and the ground resistor was extremely hot. I told them that it needed fixed before we blew up 30,000 in new drives.

The service is a 4,000 amp 480 volt 3 phase 3 wire system. On their 1 line diagram it shows the 25kv feed from the utility through metering service disconnect then feeding the transformers. It shows a delta primary and a wye secondary but only 3 wire system. I am not very familiar with this type of distrubution. Maintance did turn off as many loads as they could but could not clear the fault.

Also the ground indicator lights have been unhooked from the system, I hate to call osha but they not wanting to fix it means that I can not fix the cranes and over the 4 hours I was there the fault got worse. It want from 20 volts to ground to 35 volts to ground.

Can anybody recommend a good reference to learn more about this kind of system. Also would you notify osha or somebody else?

Sent from my SM-N900V using Tapatalk
Click to expand...

The primary side of your system from the utility is 25 kV supplied to "transformers" which suggests more than one transformer bank. I will assume both are 3 wire, WYE, 480 Volt supplies. Open the 480 volt main CBs. Check the voltage to ground on each phase and then the phase to phase voltages, A-G, B-G, C-G, A-B, B-C, C-A. Assuming a balanced supply voltage from your utility you should read 277 to ground on all three phases and 480 volts phase to phase on all three voltage. This should be true on both or all sets of transformer secondarys. If you do not get this then there is a ground, high impedance or otherwise, on one or more phases. Close the main CBs and repeat the check. When the phase to ground voltage on say B-Phase becomes lower than the 277 volts by at least 30% or so, there is indeed an indication of a developing ground. If it happens on the opposite transformer services, but on say A-Phase to ground, you may set up a ground current loop.

If the hoists are supplied by two separated transformers, a ground on one phase of say transformer bank one and another on the secondarys of transformer bank 2 supply may feed through equipment grounds on the hoists heating them to a possible failure. In any case you should find it as it is a safety issue for the workers. Also, voltages going from 25 volts to ground to 35 volts to ground indicates less of a ground condition, not more of one.

Hope this helps,

Newton1Law
 
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