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Topic Title: Construction Site RCD Tripping at CT chamber
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Created On: 27 March 2013 10:21 AM
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 27 March 2013 10:21 AM
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zimzimmer

Posts: 3
Joined: 27 March 2013

We have a subbie who uses a large 3 phase pump working on a 90A load, there was a fault on the PUMP which in turn tripped the main RCD in the UKPN CT Chamber (consumer side).
Insulation Resistance Test carried out at pump to prove we had a short between Earth and 3 Phases. Fault Identified!

Problem is why did this not trip the MDU RCD that fed the the pump?

It went through the MDU RCD feeding the Pump , then through The main MDU RCD to Trip the UKPN CT chamber RCD?
This caused the whole site to go down, really do want to prevent this from happening again.

Potential Causes maybe??

Serious Earth Leakage?
Blakley incoming MCCB's and Variable RCD settings incorrect?

Any Queries much appreciated.
 27 March 2013 10:53 AM
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AJJewsbury

Posts: 10977
Joined: 13 August 2003

Two immediate thoughts -

1. The downstream RCD was faulty.

2. There wasn't proper discrimination between the two RCDs - to discriminate, not only should the supply side RCD's non-trip current be higher than the downstream RCD's operating current (i.e. usually at least twice the nominal rating), but the time delay on upstream device be longer than the downstream device's maximum overall trip time.

We'd probably need more information for a better guess.

- Andy.
 27 March 2013 11:20 AM
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perspicacious

Posts: 7032
Joined: 18 April 2006

As Andy posted, we need details of the RCD settings starting with the incoming one and whatever is between that and the load that caused a problem. Typically each RCD will have adjustable mA and time delays with 0, 60 and 120 mS being commonly used. More sophisticated types will have a greater range.

Regards

BOD
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