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

 

[Guest123]

What circuit was the fault originating from???

The only MCB's that would operate by themselves under fault condiitons with an earth fault loop impedance of 2ohms or more would be 6A & 10A, B&C type, and 16A & 20A B type.

The 2 ohms would be too high for any of the other to operate & clear the fault. Whomever removed the RCD is a total idiot that has risked the lives of ALL staff & customers of this pub.

I can only hope you rectified the situation AS SOON AS you found it and it isn't still in this condition!

If you look at the tables in BS 7671 you will see the max values for MCB operation under earth fault conditions.

 

[Deleted member 26818]

With earth faults, there are those with negligible impedance, and those where there is an impedance.
The higher the impedance, the lower the fault current.
The lower the fault current, the longer the time a CPD will take to operate.
You don't mention what sort of device, or the rating of said device that was protecting the faulty circuit.
It appears likely that the fault had such a high impedance, that the current produced, was not enough for the device to operate.

 

[malcolmsanford]

Pesky things those RCDs, well they just keep tripping all the time, and the worst possible times, when the pub is full etc, after all it is only a small fault .....................best to get shot of them, and low and behold problem solved

I'll get my coat

 

[Guest123]

Really boils my **** does this, it's exactly as you describe Mal, some incompetent fool has been in to have a look at this problem decided removal of the RCD was the easiest way out and left it at that.

It's a TT system for gods sake, might aswell just give him an MP5 and send him on his way!

 

[johnboy6083]

it was proably done by someone who drinks down the pub. Yeah nmate, i know how to sort that out.
A decent sparks may well of offered a day rate to find the fault and rectify it, but this bloke just removed the RCD for the cost of or indeed a few pints!

I know a few blokes will moan at me saying this, but if we were to follow E54's advice and install decent earth rods with a stable low value, then this type of incident would be less likely to happen.

 

[Engineer54]

it was proably done by someone who drinks down the pub. Yeah nmate, i know how to sort that out.
A decent sparks may well of offered a day rate to find the fault and rectify it, but this bloke just removed the RCD for the cost of or indeed a few pints!

I know a few blokes will moan at me saying this, but if we were to follow E54's advice and install decent earth rods with a stable low value, then this type of incident would be less likely to happen.

Well thank you very much Johnboy!! lol!! You know it makes sense!! lol!!:hurray:

 

[johnboy6083]

Well thank you very much Johnboy!! lol!! You know it makes sense!! lol!!:hurray:

i think most sparks know it makes sense, but when there is no specific advice on Ra vales, only very vague guidance, then most people see no reason to acheive low values, when an RCD is protecting the instalation.

Its also a matter of economics, as if say i was to quote £100 for just the earth rod alobne, and mr E. Lectrician from down tghe road only charges £20 for a rod, that the customer thinks does the same job, then who will win the work?

 

[Engineer54]

i think most sparks know it makes sense, but when there is no specific advice on Ra vales, only very vague guidance, then most people see no reason to acheive low values, when an RCD is protecting the instalation.

Its also a matter of economics, as if say i was to quote £100 for just the earth rod alobne, and mr E. Lectrician from down tghe road only charges £20 for a rod, that the customer thinks does the same job, then who will win the work?

I know the problems, ....and TT installations have never been cheap to achieve when done properly!! Even the other way round the problem, is expensive. ...ie, a CU with a 100mA front end S type RCD, with RCBO's protecting all the individual final circuits. At least you'll need 2 RCD devices to fail then!!

 

[La Poste]

The ZS reading I was getting was around 2 Ohms.

This reading is incredibly low for a TT system.

 

[johnboy6083]

This reading is incredibly low for a TT system.

this will no doubt be down to the main bonding. id be suprised ig the rod was that low

 

[Guest123]

This reading is incredibly low for a TT system.

Parallel paths will no doubt have been included in the test as he states he tested Zs.


Could do with a bit more info from the OP though, quite a lot of assuming going on here.

 

[needasparks]

This reading is incredibly low for a TT system.

Does sound low but i guess there is a bit of water and gas pipes giving it a helping hand along the way.

 

[malcolmsanford]

That was the Zs for the circuit, there may have been bonding and parallel paths to bring it to that figure, but it as Lenny pointed for achieving disconnection of a protection device it is high and limited

 

[La Poste]

Still with a fault current of 230/2 = 115 Amps I'm surprised something didn't trip out.

Maybe the bloke down the pub has taken the MCB's out along with the 100ma RCD just for good measure.

 

[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.