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Thursday, 21 August 2008 |
Michael writes: I was called to a 25-floor condo in uptown Toronto to replace a broken disconnect for an elevator unit. The disconnect was connected to a splitter in the elevator machine room on the roof. This splitter was tied to an emergency power generator; this way, the elevator would continue to run in case the building lost power.
The splitter was a 120/208V 400A feeder. The feeder cables were single-conductor teck.
There was also rigging cable left near the teck for moving heavy parts of the new elevator. As I started to install the disconnect, I kicked one of these cables into the splitter feeder cables and saw some sparks.
After seeing this, I took out my meter and set it to Frequency. I put one lead to the steel of the catwalk and the other to the outer sheath of the feeder cables. As the meter shows, the frequency was all over the place, right up to the highest of 900Hz! Further investigation showed the building, elevator catwalk and feeder cables all had small—but harmful—potential between them.
The cause of the different frequency were the new elevator variable speed drives, which were replacing the old logic control. Bad grounding was the cause of this high leakage frequency and subsequent sparks.
(Video courtesy Michael Kelly of The Electrical Department Co.)
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Anthony Capkun
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House of Horrors
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