Line to earth fault but MCBs will not trip..Why?

[timmy123]

I was called to a single phase installation in a pub as the staff were receiving shocks when touching equipment such as fridges water pipes etc. I located a damaged cable where line and PE conductors were touching and causing 240 volts on the entire earthing system at the installation. In other words there was 240 volts PD between the casing of equipment/ water pipes etc and true earth. This was a TT system where the 100ma RCD protecting the whole installation had been removed at some point previously! The ZS reading I was getting was around 2 Ohms. The mystery to me is why none of the MCBs on the installation were tripping out?
I am probably missing something really obvious. Any thoughts/ ideas would be greatly appreciated.
Many thanks

 

[timmy123]

Lenny,

Many thanks for your informative reply. The fault was located in a 16mm T/E cable supplying a second DB from the main DB in the installation. The second DB had a variety of 32/20/and 6 amp MCBs. None had tripped. No RCD on second DB. The main DB had a similar variety of MCBs. All MCBs are type B. No RCD on first DB. I resolved the problem by removing the supply cable to the second DB. The pub is closed and will be until a new supply cable is run to the second DB. Thanks again for your comments

 

[johnboy6083]

no wonder it didnt trip if there was no RCD at the supply end of that cable. a 50A breaker will require 250A to trip in 5 secs, a 63A needs 315A.

 

[timmy123]

Hi Spinlondon,

Thanks for your reply and the information about low/high impedances. many thanks.
The fault was in a 16mm t/e supplying a second DB from the main DB. Both DBs had a variety of type B MCBs but no RCDs. 6,20, 32 amp MCBs. The only RCD on the installation had been removed and it was a 100ma unit located at the DNO incomer point. The pub is closed until a new RCD is installed and a new supply cable from DB1 to DB2. DB2 has been isolated by disconnection the supply from DB1
Cheers

 

[Guest123]

Lenny,

Many thanks for your informative reply. The fault was located in a 16mm T/E cable supplying a second DB from the main DB in the installation.

There we go then, large OCPD's need low loop impedances to disconnect under fault conditions which you dont ordinarily get on a TT system hence the requirement for an RCD.

Take that RCD away and..........

 

[netblindpaul]

Why are these sorts of issues occurring, this is an appalling reflection on the trade.
The perpetrator & the customer should both be prosecuted.

 

[mickytuff]

seems too low to me, theres more to this fault than meets the eye as it were.