In a recent project, a new switchboard was installed and "bus" connected with approx. sixty five feet of 3000A bus way that terminated into the new switchboard. In the process of installation the ground flange was compromised and was accidently connected with a phase in the switchboard when energized a ground fault occurred and created sever damage to the new switchboard (I will post pictures as soon as I am able to right now I cannot). The interesting part is that this was "tested" by a third party prior to energization. The test reports a megger at 1kvdc with readings between phase to phase and phase to ground and neutral greater that 2 gigaohm? why gigohm and not megaohm? and if the test was done properly why was this condition not discovered during the test? curious on your thoughts and opinions.
Bus Conn. Incident?
- Thread starter Mike01
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zbang
Senior Member
- Location
- Roughly 5346 miles from Earls Court
The test may have done correctly and the damage occurred (somebody 'modified' the bus structure) after or maybe it wasn't actually tested at all.
broadgage
Senior Member
- Location
- London, England
Most likely, either the test results were faked, OR the phase and ground were very close but NOT touching when the test was done.
An air gap of less than 1mm will easily withstand line voltage or an insulation test.
Such a small gap could easily be closed later by thermal expansion/contraction, building structure setllement, or a trivial knock or blow to the equipment.
Some insulation testers read up to G ohms.
An air gap of less than 1mm will easily withstand line voltage or an insulation test.
Such a small gap could easily be closed later by thermal expansion/contraction, building structure setllement, or a trivial knock or blow to the equipment.
Some insulation testers read up to G ohms.
rcwilson
Senior Member
- Location
- Redmond, WA
Was the test done just prior to energization? Could the test have been done on just the switchboard prior to the final bus connections? Was any work done after the test and prior to energization?
When I was doing testing, to meet the contractor schedule we had to checkout the gear prior to completion many times. We would strongly recommend that additional testing be done just prior to energization.
Is the bus fed by switchgear or is it direct from a transformer or service? If the bus is connected to a transformer the neutral to ground connection at the transformer and the transformer windings make phase-phase and phase-ground meggar tests read 0 ohms. The fault would not get noticed unless the bus is completely unbolted from the transformer. The test technician may have lifted the neutral disconnect link and just tested the switchboard bus downstream of the main. (Injecting a low voltage AC signal phase-phase and phase-ground is one way of testing the bus. If you can build some voltage, it's clear. If the test set hums at 0 volts, look some more.)
When I was doing testing, to meet the contractor schedule we had to checkout the gear prior to completion many times. We would strongly recommend that additional testing be done just prior to energization.
Is the bus fed by switchgear or is it direct from a transformer or service? If the bus is connected to a transformer the neutral to ground connection at the transformer and the transformer windings make phase-phase and phase-ground meggar tests read 0 ohms. The fault would not get noticed unless the bus is completely unbolted from the transformer. The test technician may have lifted the neutral disconnect link and just tested the switchboard bus downstream of the main. (Injecting a low voltage AC signal phase-phase and phase-ground is one way of testing the bus. If you can build some voltage, it's clear. If the test set hums at 0 volts, look some more.)
Test Plan?
Test Plan?
Did they have a test plan?
It's a good ideal to require a Test Plan submitted and reviewed before the testing takes place. The Plan should include any drawings necessary to define the scope of the test.
Good Testing outfits do this as a matter of course. The not so good ones send out some guys with a meggar.
A one line drawing is essential, it's the map of the plan. "IF you don't know where your going don't be supprized if you wind up somewhere else."
Test Plan?
Did they have a test plan?
It's a good ideal to require a Test Plan submitted and reviewed before the testing takes place. The Plan should include any drawings necessary to define the scope of the test.
Good Testing outfits do this as a matter of course. The not so good ones send out some guys with a meggar.
A one line drawing is essential, it's the map of the plan. "IF you don't know where your going don't be supprized if you wind up somewhere else."
brian john
Senior Member
- Location
- Leesburg, VA
IMO,
The best way to handle busway
Megger each piece as it comes off the truck.
Megger each piece as it goes up then megger the assembled run, repeat for each piece.
Then megger per NETA at completion, I have never seen what you say happened, happen.
The best way to handle busway
Megger each piece as it comes off the truck.
Megger each piece as it goes up then megger the assembled run, repeat for each piece.
Then megger per NETA at completion, I have never seen what you say happened, happen.
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
What excuse does this "third party" give for the fault and resulting damage?
zog
Senior Member
- Location
- Charlotte, NC
Well Gig is 1000x Meg so not sure why you are queationing that, but there are 2 possible senarios here, most likely the tests were done prior to final terminations, as is often done, or the tests results are BS. Now I personally know every testing company in Michigan well, most are NETA and most know what they are doing so I doubt this is the issue.
How about a bad lead on the megger. When making resistance measurements and checking for open circuits or high megger values do you know the connections are good. A poor connection/bad lead looks just like an open circuit. A simple method is to connect a voltmeter on the bus somewhere else. If you are using a 1000volt megger put a voltmeter that can read 1000 volts on the buss and see the 1000 volts the megger produces. Remove the volt meter before you make the final test, or you will determine the input resistance of your volt meter. This simple test insures that all the connection are good to the bus and no probes not in all the way, broken wires, high resitance connection, etc.
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