V.Low IR readings help please!!!!!!

[installav]

Hi Guys,

hope someone can help, I am newly qualified and on my first job on my own I have just added a spur to an existing ring, I was happy with the installation until testing.

Carried out 500v IR test between E-N and E-L and getting readings of 0.004M. The building is a large old victorian building with steel tubes run everywhere which provide the earth to all circuits therefore at the local CU there are no earth tails to remove so i cannot isolate the earth circuit from the ring i am working on, would this cause the low readings?? I tested a circuit that i hadn't even worked on and got the same readings. I verified the meter with a checkbox and the meter is spot on?

Would anyone have any ideas on what the problem may be caused by or what i could do to investigate further,

many thanks

Wayne

 

[tony mc]

With it being the building as discribed did you test the IR before you started the work ?

 

[malcolmsanford]

Is the cable old VIR ?

Your IR results should not be influenced by parallel paths. did you take a LIVE/LIVE test?

Did you do a continuity of protective conductor test ie with a wander lead throughout the installation to test for continuity of even if there were earths at various points?

 

[tony mc]

As Malcolm has indicated.

I would disconnect both the line and neutral from the CU but make sure that everything is diconnected from the circuit.

I would but money on that something is still connected test betwenn live conductors with your tester on Ohms and see if anything is connected!

The reading you are getting is 4000 ohms so I presume the RCD if installed will not hold.

Try a soft test on 250 V on the disconnected cct and see what your readings are!

 

[installav]

With it being the building as discribed did you test the IR before you started the work ?

hi toni,

no i didnt and really wish i had, probably wouldn't have touched the job, as i don't think what i have done has any bearing on the readings. just worried now about possible future problems. what i find strange is that no RCD'S trip even with these readings, neither the RCD socket i have installed nor the RCBO for the circuit, not even with lots of electrical equipment plugged in and working..

 

[installav]

Is the cable old VIR ?

Your IR results should not be influenced by parallel paths. did you take a LIVE/LIVE test?

Did you do a continuity of protective conductor test ie with a wander lead throughout the installation to test for continuity of even if there were earths at various points?

hi malcolm,

didn't do a L/N test as there are pc's and equip. plugged in everywhere, someone else did suggest to me joining the L and N tails and retesting. may be a case that i have to walk the floor and disconnect all the equipment..

i thought maybe i had been thrown a curveball with the earthing system

 

[tony mc]

You need to unplug everything from the cct that you are testing or it could be a piece of equipment giving you false readings. Join L/N together & test to earth .

AS Malcolm has said test for continuity of protective conductor first.

Are there any surge protective devices fitted ?? Hence the soft test !!

The RCBO and SORCD on the same cct ??

Did you do a ZS if the cct is up and running what was the reading?

I would start from scratch and go through your testing procedure making sure that before you move to the next test you are happy and the readings are acceptable.

What is the earthing at the house?

 

[truckster]

With all the PC's connected you are going to get low IR readings to earth because of the functional earths. The sockets may be only single pole so by simply turning them off you aren't isolating them completely you will need to actually unplug them.

 

[installav]

hi toni,

the rcbo and the sorcd are on the same circuit, the zs value was 0.55. the earthing system is tn-s.

not sure about any surge protective devices..

really appreciate you and malcoms help with this.

 

[tony mc]

I think you will find that once you have disconnected all equipment and test the cct you are working on removed from the CU your readings will go up.

With the RCBO holding I will put money on the equipment pluged in or a surge protector etc affecting your readings.

Take your time when you go back and make sure eveything is disconnected before putting 500 V on L & N start with a soft test and see what it reads!!!

I can't see the point of putting a SORCD on the same cct as as RCBO if they are both 30 mA.

Always best to do a small amount of testing prior to starting any work for obvious reason!!

Good luck!

 

[Dunc]

If the Low readings aren't coming from the PCs (which is incredibly unlikely) then it'll be from your RCD socket. All loads should be removed from the circuit before IR tests are carried out but at the very least I verify that they are by carrying out a 250V test between L+N and E.

 

[mark12spark]

Re: V.Low IR readings help please!!!!!

Hi. Have you tried L1 and N shorted together to earth? Sounds like there is an appliance in circuit to me. On your comment about an RCBO protecting the circuit and installing an RCD protected socket, this is not usual as you should avoid having two RCDs in series unless one is higher mAmpage or time delay. Say 100mA upstream and 30mA at the outlet..

Cheers

Mark

 

[tony mc]

Dunc has made a good point re the socket RCD over the last few months we have encounted problems with these on IR testing make sure that both line conductors are disconnected from the socket !!

 

[MarkieSparkie]

Safety socket RCDs are electronic devices and should be disconnected when IR testing. If left in circuit the IR test is likely to forward bias or breakdown semiconductor junctions leading to the type of results you are seeing.

 

[installav]

Re: V.Low IR readings help please!!!!!

Hi. Have you tried L1 and N shorted together to earth? Sounds like there is an appliance in circuit to me. On your comment about an RCBO protecting the circuit and installing an RCD protected socket, this is not usual as you should avoid having two RCDs in series unless one is higher mAmpage or time delay. Say 100mA upstream and 30mA at the outlet..

Cheers

Mark

Hi Mark,

I totally agree, however the company we are doing the work for are insistent that we use Socket RCD's regardless. I think they see it as a belt and braces situation.. I take onboard your advice however on the higher rating upstream.. Many thanks, wayne