1st post - be gentle...

We have completed a job, but during testing we have found a problem on several circuits. An example is this; Radial circuit wired in twin and earth has too high a Zs reading, the circuit is protected by a 61009-2 rcbo.

I have spoken to a colleague who says that instead of a phase - cpc test i can do a phase - neutral test. As the circuit has a larger neutral than cpc, this brings down the 'R1+R2'value. He says this is due to rcbo protection.

Question 1 - is he right? Question 2 - if he is, how do i record this on the test certificate?

Regards....

Hi,
You need an earth for this circuit. So how are you going to prove you have one by testing phase to neutral? I'll let you think about that one. The answer to your question is no he is not right.

That doesn't sound ok to me, maybe you should find a way to reduce the Zs by reducing the circuits R2?

So your friend is quite happy to have a non-compliant circuit put into service? Strange.

-------------------------
Regards
Carl

Chris

There is an earth - and it has been proved. The problem is that the resistance of the earth causes the R1+R2 to be too high.

I'll be honest, i actually confirmed this with the NICEIC technical helpline, however when i called back to ask about the test cert, i was told they finish early on a Friday!

It does sound a bit dubious to me, however i can't think of a practicle resolution.

CB


Practical - so that's what that re underline was..... Thanks Patt2

Edited: 23 October 2009 at 08:31 PM by currantbun

Dont you mean the Zs, if not, what is the R1+R2 reading?

Can you not wire in an extra CPC, adequately sized for the circuit? I dont know what the circuit is?

-------------------------
Regards
Carl

Sorry about that ,Gremlin.
A practical solution would be to re-design the circuit to give an acceptable R1+R2. A way to do this would be to increase the size of the cpc.

There are various circuits, all with the same problem. One of the worst ones is external lighting, it's all been concreted now so there is no chance of running in a new cpc.

The main problem is the Ze being high (0.11), but because i can't change that i need to bring the R1+R2 down. The circuit is wired in 2.5mm twin and cpc. If the cpc was the same size as the phase and neutral there would not be a problem.

CB

What earthing system have you been working on? What earthing system would consider 0.11 ohms to be high? None that I can think of?

TN-S, 0.80 Ohms Max
TN-C-S, 0.35 Ohms Max
TT, 200 Ohms Max

Where's the problem with 0.11?

-------------------------
Regards
Carl

Sorry Carl, what i meant was that i cannot alter the Ze. I know that it is not particularly high.

The issue is with the Zs, the Ze and R1+R2 together are too high and i was wondering if anyone had previously came across the suggestion that i'd been given.

What size is the RCBO overload protection? If its lighting could you bring it down to a 10amp or even 6 amp breaker- that would surely give enough R1/R2 to play with other wise it must be a very long circuit if you are getting a rubbbish Zs with a circuit in 2.5 t&e. Have you checked connections along the circuit- maybe its as simple as a poor connection somewhere- why not measure R1 and R2 seperately to pinpoint the problem? Just a few extra thoughts.

I may be wrong but this sounds kinda DIYish to me...

Firstly its an RCBO,and u r asking about testing the neutral for an earth path... not thinking of strapping them together surely?

First thing I would ask is what's your R1+R2... Some 2 lead testers will read <1ohm resolution for RCD protected circuits... much to my amazement even! So is your R1 + R2 many ohms?

When finding higher R2 values consider the obvious to overcome but look also at volt drop of R1. Consider also .. no I won't say that one..

-------------------------
Online Services - http://propertydevelopment.org.uk

Experience can sometimes show that cost prevails over quality and safety, such little self-value that people hold.

A rcbo 30mA rated has a max Zs of 1667 ohm so whats the problem? You do not have to worry.

-------------------------
James

I also thought the same thing, but didnt feel right about saying it. Would anybody accept a reading of 1667 ohms, not that you will ever see it in all honesty?

-------------------------
Regards
Carl

Agreed the tester would have to be concerned if a readinganywhere approching that value. My point is if the R1R2 or Zs is a little high the rcd element of the rcbo will clear the fault instead of the magnetic element.

Earth loop values are not as critical with RCD/RCBO devices.

-------------------------
James

An RCD cannot be considered as a catchall for such discrepancies, the values of R2 require to conform to the regulations, the OP demonstrates incompetent design for a new installation......oops!

Regards

Interestingly, the use of an rcd to meet disconnection times in the event of an earth fault is inherently unreliable. It has already been ascertained that at least 3% of rcds do not trip under fault conditions, possibly due to mechanical issues. I cannot see the sense in abandoning a reliable system for a less reliable one - unless you take into account the eagerness of the rcbo/rcd manufacturers to push for the acceptance of Type C rcbo`s for standard domestic use to save THEM money of course...................

To replace a 6 way fuseboard on a 5 bedroom property, I've just installed a 14 way DB, all with RCBO's - a Steeple lesser variety, (I know, I know.......lucky I had a shoehorn in the toolbag for the cable)

Instructions are not to carry out a Zs nor IR test on RCBO's, this changes the testing procedure now...Thinks, thinks.

Regards

Quite correct not to IR test an RCBO,but you should still test the cables,and Zs can be calculated,so i dont see where the testing procedure has changed.

-------------------------
Regards

Dave(not Cockburn)

We use RCDs to compensate in TT installs for the fault current. It doesnt compensate for long circuits for short circuit currents and volt drop should also be considered. Circuit R1+R2 values should be within normal parameters for ADS during initial verification. If carrying out periodic repairs in an existing installation it may be an option to add an RCD if not already present to give a cheap option to compensate for general deterioration raising values until adequate repair can be made. Raised values I would also be concerned about poor contacts and fire risk. So using RCBOs as a first port of call without proper informed consideration is a dangerous option in my view.. (IMHO)

-------------------------
Online Services - http://propertydevelopment.org.uk

Experience can sometimes show that cost prevails over quality and safety, such little self-value that people hold.

Whats to say a mcb wont have the same mechanical problems as an rcd its just most people dont have a method of testing them. I dare bet there are 1000's of faulty MCB's out there "PROTECTING" there owners.

Market forces have made manufacturers to supply cheaper and cheaper products and we are now able to buy mcb's for under 2 quid and a rcbo for under 20 but whats the quality like? and how are they going to fare over the realistic 30 year life span we expect?

-------------------------
James