This Has Happened Before

[brian john]

Had a job Tuesday evening, building flooded and busway was in the path of the water, no dams around the busway chases between floors. I informed the customer if I meggered the busway and the readings were unacceptable I would not re-energize the busway. This system had been on line 30 hours since the flood and at no point had been de-energized.

Powered down the busways 2000 amp and 1600 amp.

Meggered at 1000 VDC, A Phase-163,000 OHMS and it was the same for all phases to ground and phase to phase, both busways. triple checked all busway switches were off and looked for any busway taps, all was off. I explained to the customer and told them I would not re-energize as previously explained and told him I could not recommended he re-energize. Long and short a temporary was installed, bypassed the wet/defective portions. The portions of the busway removed had water pouring out of them. The portions left intact meggered in excess of 2 gigohms.

Now this busway was up and running with no issues other that a possibility of blowing up at any point.

Had a 45 kva transformer (another site) under sewage and other that the steam it had output voltage and the customer was using this to operate pumps.

Had a busway that had been tripped off due to a UV operation and was reset, at one point we were hired to megger it readings were in the 1-2 megohm range. Busway was shut down and a 90 was located with an accumulation of rust, filings dirt muck and evidence of past water intrusion.

Has anyone else been involved with distribution equipment that should obviously have been damaged and should have operated protective devices but didn't?


Any thoughts on why?

 

[nakulak]

only reason I can think of is: after a building is energized, no maintainance is ever performed on the service until something blows up.


(did all this equp have ground fault protection ? how old was this stuff? I've come to believe that the newer state of the art services with the ground fault monitors and phase protection were pretty nice, but I never see them again (mostly) - are you saying this new equipment is junk with a false sense of security ?)

 

[winnie]

Why do you say that protective devices should have tripped?

Clearly there were real problems, which have a very real possibility of escalating in a catastrophic fashion.

But think about what a 160,000 ohm fault means. At 480V, only 3mA will flow through 160,000 ohms, dissipating 1.5 watts. This low insulation value tells you that the insulation is damaged, and that repairs are required. It tells you to expect that things will get worse, perhaps much worse very quickly, but the magnitude of the problem _at that moment_ is pretty small.

A surprising amount of equipment will work for a surprising long time, totally immersed in dirty water.

-Jon

 

[zog]

Had a job Tuesday evening, building flooded and busway was in the path of the water, no dams around the busway chases between floors. I informed the customer if I meggered the busway and the readings were unacceptable I would not re-energize the busway. This system had been on line 30 hours since the flood and at no point had been de-energized.

Powered down the busways 2000 amp and 1600 amp.

Meggered at 1000 VDC, A Phase-163,000 OHMS and it was the same for all phases to ground and phase to phase, both busways. triple checked all busway switches were off and looked for any busway taps, all was off. I explained to the customer and told them I would not re-energize as previously explained and told him I could not recommended he re-energize. Long and short a temporary was installed, bypassed the wet/defective portions. The portions of the busway removed had water pouring out of them. The portions left intact meggered in excess of 2 gigohms.

Now this busway was up and running with no issues other that a possibility of blowing up at any point.

Had a 45 kva transformer (another site) under sewage and other that the steam it had output voltage and the customer was using this to operate pumps.

Had a busway that had been tripped off due to a UV operation and was reset, at one point we were hired to megger it readings were in the 1-2 megohm range. Busway was shut down and a 90 was located with an accumulation of rust, filings dirt muck and evidence of past water intrusion.

Has anyone else been involved with distribution equipment that should obviously have been damaged and should have operated protective devices but didn't?


Any thoughts on why?

All the time, of couse it wont blow up right away, do the math. 480V/150 k ohms = about 3.2 mA, so no protective action is expected, but what is happening is tht 3.2 mA is traveling through the insulation so do the math backwards. 3.2mA squared x 150k ohms = 1.5 watts which arcoss a small distance is a fair amount of heat and current to break down the insulation further, look at this from a energy point of view and in 1 month you have dissipated over a kW of energy, in a year almost 15kW across a few inches of insulation, that causes breakdown of the insulation and a failure (Eventually). Will it blow up right away, probally not, but when will it be tested again? Answer, not before it fails. I assume your posted values are tempature corrected (Cause I know you are the man), so in reality in service IR is lower than you posted, add humidity or egress of moisture/contaminates to the equation and the failure happens sooner.

We have a form that we ask the customer to sign if any test results are less than our (NETA) specs that acknowledges they have been advised not to energize and are doing so against our recommendations.

 

[zog]

Dang winnie, you beat me to the punch, I guess I am too long winded.

 

[brian john]

System had GFP. But like a transformer operating under sewage.....

 

[masterinbama]

I have seen this with old lead and paper sheathed 5KV cable. As long as it was energized it was okay,but as soon as we shut it down for any length of time it would fail a megger test.

 

[zog]

I have seen this with old lead and paper sheathed 5KV cable. As long as it was energized it was okay,but as soon as we shut it down for any length of time it would fail a megger test.

Thats paper for ya