Newbie Insulation Resistance question

[DaveS]

Hi all hope everyone's had good weekend.

I am currently studying for 2394/5 and and getting used to testers/testing by doing abit around the house. Where the insulation test is concerned i still have a few questions. I have a 2 way lighting circuit in living room and although i know that i should test both switches to include strappers. The light fitting is 10*12W and has a transformer. Will passing 500v through this potentially damage it? if so how can this circuit be safely tested without taking the fitting down. I also have a SELV shaver socket in bathroom and although i have read (maybe mistakenly) on other threads that i need to just disconnect at board and connect L and N together and test between these and earth. Is this still not going to pass 500v through SELV socket. Also been advised to switch light off in bathroom but i wouldn't think this makes any difference as SELV socket is just looped off light anyway.

Cheers

 

[davesparks]

If it's a new installation you need to disconnect the light and the shaver socket.

If it's an existing installation you have to use your professional judgement to decide whether joining L and N and testing to E only is acceptable.

And I very much doubt that your shaver socket is SELV, you don't get many electric shavers that work on 50volts or less!

 

[DaveS]

Thanks for response davesparks. I should of said its an electrically seperated shaver socket/light fitting. Fed from bathroom lighting circuit. Because this is an existing installation if i was recerting i understand this would be an EICR. What are acceptable tests in this situation without disconnecting all vunarable equipment. I acknowledge you said need to use judgement but is there some sort of guidance on acceptable recordings? Also the other thing i can't fully get head around is that even if disconnect from board and connect L&N together and test between these and earth am i not still putting 500v through any fitting. Probably missing something obvious but why does connecting L & N together make any difference as in other posts people seem to suggest that is does?

Any replies would be appreciated p.s please be gentle

 

[Richard Burns]

For an existing installation then you can test with line and neutral connected together against the cpc (connected to the earth bar, to check for other non cpc low readings). This test is detailed in guidance note 3 and has the same limitations as an individual circuit test (greater than 1 MΩ and preferably greater than 2MΩ)

If you connect line and neutral together then you do not apply a potential difference to the functional circuitry of the piece of equipment (as both line and neutral are at the same potential) the only 500 V potential difference is from the internal (and external) circuitry to the circuit protective conductors.

Because the circuit protective conductors should not be connected to any of the circuitry then the 500 V will not be a problem (unless there is a fault, or if there are functional earths (such as on RCBOs) or if there is a power smoothing circuit that has an RC link to earth (such as in audio equipment and most computer equipment)) and even if there is a connection then the equipment should be designed to handle this voltage to earth.

Of course local surge protection devices (such as extension blocks for computers, etc.) are designed to pass the voltage to earth above 400V and so may well give false readings.

Separated equipment should be able to be tested safely at 500V, and the additional test for separated equipment should also be undertaken.

SELV equipment should be tested internally at 250V and externally at 500V, however I would not bother for any limited SELV items such as lighting transformers.

 

[PEG]

Another crackingly concise explanation,Richard :28: nice one

 

[DaveS]

Thank you Richard for taking the time with such a helpful post. Sort of dawned on me earlier (silly mistake) :89:but great to have it confirmed. Better to ask than never know plus hesitant to start testing with 500v and not know what affects it may have.

 

[7029 dave]

Good advice given to you, can't add to that.