global ir test ???

[wa209]

Hi to all..have heard some people talk about a basic '1st' test regarding a possible cu change , called a global ir test. I understand that this that this a test performed on all circuits which may assist in determining if a fault is present before delving into the indivdual circuits.
Being new , i have not done one , or been present when someone else has done one.
Can someone piont me in the right direction re method and principles behind it ?
Not that i am going out to perform tomorrow , but i might get a couple of expirienced electrians to talk me through it.
Always helps to have a bit of understanding beforehand.....

Thanks to all as usual

 

[trev]

Suppose your CU has one RCD. Remove all loads and sensetive equipment then with test probes on the load side of RCD and all MCBs in the on position send your test voltage in. Acceptable IR result is in OSG and GN3.

 

[trev]

RCD is in off position by the way.
Note to self, energise brain before energising typing fingers

 

[telectrix]

and if there's no RCD, swsitch main switch off. you perform an IR test between each L and E and then between the N and E bars. obviously you then have all the neutrals and Earths in parallel so expect a reading less than for individual circuits.

 

[Mark_Burgess]

If following the above tests make sure ALL appliances are unplugged or isolator switches off.

Alternatively for an indication of condition, link the line and neutral then test to earth.

 

[Guest112]

and after you do the above mentioned tests known as a "block" test if you have low IR readings test each circuit individually to narrow down your search for the cause of the low resistance

(youd do this first of all by turning the mcbs off and testing the load side of each circuit to earth or by removing neutrals individually and testing to earth etc)

and the reason youd do this as youve mentioned to avoid any issues installing new equipment for example if your installing rcd protection the condition of an old circuit which has only previously had fuses/mcbs the leakage will affect the rcd and trip it if there is a problem with the circuit etc

 

[tony mc]

Hi wa209

If you do perform a globle IR test as mentioned above be aware of any surge protection devices plugged in around the premises when testing at 500 volts as they will see the high voltage and dump it to earth giving you a false reading on your test.

IMO I would always soft test at 250 v in the first instance .