High earth loop readings

[raymond11]

Hi I am getting some high readings on lighting circuits of around 2.97 ohms ,the protective device is an rcbo/ combined Afd .I am getting trip times of 34ms x1 and 9ms X5 .the maximum value, looking at the pocket table ,the maximum should not exceed 1.74 ohms for a 10 amp rcbo.

Any ideas ? Not sure about the 1667 ohms deal ,if there is a reg to confirm this is ok ,could you please attach to your replies.

Many thanks

 

[SPECIAL LOCATION]

Initial thoughts...

I am assuming this is a 10A type C..?    (60898 / 61009)

i.e. Max 2.19,    80% rule of thumb 1.75..   hence your 1.74 pocket guide?? 

Do the circuits need to be type C ?

as a 'B' would give a bit more tolerance..  Max 4.37  (80%  3.49)..

Is this a new circuit?

or

testing existing wiring?

if existing.. do you know what previous test readings said?

What are the actual R1+R2 values? 

do they seem appropriate for the cable CSA & circuit length..?

Possibly a bit more investigating could reveal a poor joint somewhere?

especially if it is an existing circuit that had previous good readings..

:C  

 

[Murdoch]

I'm very intrigued

A combined RCBO / AFDD

Please expand on this point

 

[Fleeting]

What is deemed high as your reading may be acceptable for the circuit length.

 

[Sharpend]

I'm very intrigued

A combined RCBO / AFDD

Please expand on this point

https://www.google.co.uk/search?

 

[Murdoch]

https://www.google.co.uk/search?

I'm just gob smacked that any are being installed .......

 

[Sidney]

What's your Ze?

What's the R1+R2?

Three lead/no trip tests often give higher results due to a lower test current. There's also RCD uplift to take into consideration.

It is acceptable to add your measured R1+R2 to your measured Ze for your Zs. But you could always carry out the alternative Zs test:

• Safely isolate the installation.
  
• Have main switch off, all MCBs off and all RCDs off.
  
• Link line and CPC at the furthest point on circuit.
  
• Use a two lead high current earth fault loop impedance tester.
  
• One probe goes on the outgoing side of the MCB of the circuit that you have linked out (and so is connected to earth via the line).
  
• The other probe goes on the incoming line conductor of the main switch, which will be live.
  
• Measure the Zs and record.
  

 

[kerching]

Will an AFDDEFGHIJKLM or whatever they are calling them actually work on a lighting circuit....genuine question. As I believe they have a minimum amount of current to operate which is only found on ring mains? NB they may now be called RFcs or some such twaddle, I'm old, deal with it

 

[Fleeting]

For them to operate on a ring final circuit, ignoring any radial spurs from it you will need two faults because a single poor connection is not likely to produce an arc. 

 

[Geoff1946]

For them to operate on a ring final circuit, ignoring any radial spurs from it you will need two faults because a single poor connection is not likely to produce an arc. 

Should they not detect an arcing fault in an appliance plugged into the ring?

 

[Fleeting]

Should they not detect an arcing fault in an appliance plugged into the ring?

Yes that is one of their anticipated benefits but an issue in the core of the circuit is not likely to be detected by a single point fault.