TT Ze reading- loop high or loop no trip?

[South Spark]

When measuring Ze on a TT that's RCD protected which is more accurate? Getting 27.1 on high current but 563 on no trip.

 

[Gazthesparky]

When measuring Ze the measurement should be taken before the RCD on the supply side with the main earth disconnected from the Met in this case directly to the earth rod. with no RCD in line you should test on the high setting as there is no RCD to worry about.

 

[Octopus]

OP - where were you measuring the Ze?

 

[davesparks]

Hi current testing should always be used for EFLI tests, except where this is made unfeasable by the presence of an RCD when a no-trip test might be used as an alternative.

Hi current testing should be your default method,

 

[South Spark]

The TT is an offshoot of a PME system. The 6mm T+E supply is on 32A RCBO at the main CU and earthed to the MET.

I've installed a mini DB also Rcd protected but earthed it to rods. Obviously when I test high current (at the mini DB) it takes out the RCBO but gives the value 27.1. On loop no trip I get the very high value of 563.

 

[westward10]

Why is the supply to the mini DB on an rcbo surely you have lost discrimination does it require this additional rcd protection.

 

[davesparks]

The TT is an offshoot of a PME system. The 6mm T+E supply is on 32A RCBO at the main CU and earthed to the MET.

I've installed a mini DB also Rcd protected but earthed it to rods. Obviously when I test high current (at the mini DB) it takes out the RCBO but gives the value 27.1. On loop no trip I get the very high value of 563.

What exactly are you measuring here, the Zs of the distribution circuit or the Zdb of the sub-DB?
what is the Ra of the earth electrodes?

 

[HappyHippyDad]

Hi current testing should always be used for EFLI tests, except where this is made unfeasable by the presence of an RCD when a no-trip test might be used as an alternative.

Hi current testing should be your default method,

Christ!!!

For a moment then I thought Dave had started the sentence by saying 'Hi' to the OP. I actually stopped eating my breakfast!

 

[Toneyz]

The TT is an offshoot of a PME system. The 6mm T+E supply is on 32A RCBO at the main CU and earthed to the MET.

I've installed a mini DB also Rcd protected but earthed it to rods. Obviously when I test high current (at the mini DB) it takes out the RCBO but gives the value 27.1. On loop no trip I get the very high value of 563.

Is this 6mm T&E within the 50mm of wall surface to the mini DB.

 

[Gazthesparky]

The TT is an offshoot of a PME system. The 6mm T+E supply is on 32A RCBO at the main CU and earthed to the MET.

I've installed a mini DB also Rcd protected but earthed it to rods. Obviously when I test high current (at the mini DB) it takes out the RCBO but gives the value 27.1. On loop no trip I get the very high value of 563.

Installing a up front RCBO means that you have lost discrimination in the 2 RCDS and it is pointless having an RCD in the mini DB as the supply circuit is RCD protected.

you would be better installing the twin and earth surface or installing an earthed metallic sheathed cable this then does away with the RCBO and just have a breaker.

This then does away with the RCD Discrimination Problem. You don't necessary have discrimination between the breakers but its a far better solution.

If you still want to use the upfront RCBO then I would suggest using a normal breaker as a temp measure to supply the mini board then disconnect the earth rod and test between that and the supply Phase using the HIGH setting on the tester. This then gives you the Ze of the mini DB (which is the Ra resistance as that is the means of earthing.

Then reconnect the supply to the RCBO and test between the earth of the twin and earth and the Phase this then gives you the Zs of the supply circuit which is supplied off of the PME supply.

using the low test always gives you a higher worse reading as its using a softer test so not to trip any RCD but as stated above a Ze measurement should be taken using the high setting with no RCDs in line.

 

[South Spark]

The introduction of the mini DB was to deal with the issue outlined in this thread

Exterior socket on PME system - http://www.electriciansforums.co.uk/threads/exterior-socket-on-pme-system.121435/

Under normal conditions there will be no discrimination that is correct but I was dealing with the issue of exporting PME.

So in the event of loss of PEN, the mini DB is earthed via rods TT. In that case during a fault I'd expect the mini DB to trip but not necessarily the RCBO at the main board.

 

[davesparks]

The introduction of the mini DB was to deal with the issue outlined in this thread

Exterior socket on PME system - http://www.electriciansforums.co.uk/threads/exterior-socket-on-pme-system.121435/

Under normal conditions there will be no discrimination that is correct but I was dealing with the issue of exporting PME.

So in the event of loss of PEN, the mini DB is earthed via rods TT. In that case during a fault I'd expect the mini DB to trip but not necessarily the RCBO at the main board.

It's pot luck which one of the RCDs trips when they are in series.

Why have you used T&E? The thread you have linked to says that the socket is to be used for events, so you are surely considering the requirements of BS7909? I'm thinking specifically of the circuit feeding the socket having a CPC of equal CSA to the line conductor and of putting the RCD in the same location as the socket.