a10jp
Electrical
- May 18, 2005
- 150
As an electrical engineer, system grounding is not an area I have too much experience with. We had a situation with the user recently, where we had power outages over a weekend for a medical facility, and we have to replace some cables and breakers, and when we turned on the system to test the emergency power, some of the medical equipments were fried. Basically we are looking into what went wrong with the existing system, and see if there is any problem that can be fixed.
The user mentioned that the facility, which was built some 30 years ago, did not have good grounding. He is stating based on his experience as fixing up the place. He said sometimes he noticed that are 2~3 branch circuit sharing only one ground. And nowadays, we always specify each circuit to have individual grounding.
After some painful investigation with the contractor, I have good idea when the surge occurred (although not sure if this is the right term, because it could also be ground fault too). After the power outage, the electrician gradually turned on the building power from substation down to individual panels, and load connection. So I do not think that is where problem is.
The problem occurred when they simulate the emergency "load" test after starting up building power, where building load was "on" and they simulate the "power-down" scenario by:
1. Disconnecting the substation 600A vacuum breaker.
2. Generator detected power down and started.
3. ATS waited for about 3~8 second, then switched from commercial to standby power mode.
4. Then when they are ready to return to normal power, they turned on the substation breaker.
5. Generator detected normal power, and stopped.
6. ATS waited for about 3 second, then switched from Standby power to commercial power
7. At this point, the TVSS sounded the alarm, also some other alarm (fire sprinkler panel) was also set off.
8. Building back to normal power (TVSS still in alarm mode).
Since TVSS alarm occurred in Step 7, I believe that is where the main surge occurred.
According to what user told me, if indeed the facility does not have good grounding, which they said is buried in the crawlspace somewhere since they built this place, but no one knows, so when the power was turned on, and the ATS flipped over from standby to commercial, and since the building is on "some" load already, the surge current may not have anywhere to go. It travels back to the system somewhere. Does that make sense? This is just a theory. I would to like to go and check, but do not know where to start measuring to verify.
Another thing I observe is that there is no clear building ground grid. According to the current set of drawings I have, the substation ground is the only ground I can see. But the substation is some distance away, wouldn’t that cause voltage drop in the ground over distance. That is why we may not have equipotential at all grounding in the building. I could now see the value of a clearly defined building ground system where ground measurement will be the same throughout the facility. But again, this needs to be verified.
Another thing I observed is that there is no surge arrestors at the substation, which has 3 transformers, 2 of them are delta-delta, and the last one which supply this MEDDAC facility is delta-wye, but with nothing. I recommend one to be put in. In Japan, they often use delta-delta system in the past, and their ground system are floating, which is dangerous from NEC perspective.
The user mentioned that the facility, which was built some 30 years ago, did not have good grounding. He is stating based on his experience as fixing up the place. He said sometimes he noticed that are 2~3 branch circuit sharing only one ground. And nowadays, we always specify each circuit to have individual grounding.
After some painful investigation with the contractor, I have good idea when the surge occurred (although not sure if this is the right term, because it could also be ground fault too). After the power outage, the electrician gradually turned on the building power from substation down to individual panels, and load connection. So I do not think that is where problem is.
The problem occurred when they simulate the emergency "load" test after starting up building power, where building load was "on" and they simulate the "power-down" scenario by:
1. Disconnecting the substation 600A vacuum breaker.
2. Generator detected power down and started.
3. ATS waited for about 3~8 second, then switched from commercial to standby power mode.
4. Then when they are ready to return to normal power, they turned on the substation breaker.
5. Generator detected normal power, and stopped.
6. ATS waited for about 3 second, then switched from Standby power to commercial power
7. At this point, the TVSS sounded the alarm, also some other alarm (fire sprinkler panel) was also set off.
8. Building back to normal power (TVSS still in alarm mode).
Since TVSS alarm occurred in Step 7, I believe that is where the main surge occurred.
According to what user told me, if indeed the facility does not have good grounding, which they said is buried in the crawlspace somewhere since they built this place, but no one knows, so when the power was turned on, and the ATS flipped over from standby to commercial, and since the building is on "some" load already, the surge current may not have anywhere to go. It travels back to the system somewhere. Does that make sense? This is just a theory. I would to like to go and check, but do not know where to start measuring to verify.
Another thing I observe is that there is no clear building ground grid. According to the current set of drawings I have, the substation ground is the only ground I can see. But the substation is some distance away, wouldn’t that cause voltage drop in the ground over distance. That is why we may not have equipotential at all grounding in the building. I could now see the value of a clearly defined building ground system where ground measurement will be the same throughout the facility. But again, this needs to be verified.
Another thing I observed is that there is no surge arrestors at the substation, which has 3 transformers, 2 of them are delta-delta, and the last one which supply this MEDDAC facility is delta-wye, but with nothing. I recommend one to be put in. In Japan, they often use delta-delta system in the past, and their ground system are floating, which is dangerous from NEC perspective.