Will RCD Trip? Testing Prior To Consumer Unit Upgrade.

[acvc]

If asked to replace a BS3036 fuseboard with a Dual RCD Consumer Unit, you would have to test all circuits being connected to it etc for the EICert.

But, while doing this yesterday I have to admit that I realised that I don't know what test result would indicate that an RCD would trip if 'put on' that circuit.

R1+R2/r1+r2 I don't think would affect it - obviously the CPC must be present and continuous.


So after thinking about it, am I correct with the following?

The RCD will trip given an imbalance between line and neutral more than or equal to 30mA (or whatever it tells you on the ramp test, usually around 27mA). So if the insulation-resistance tests shows that there is a difference of more than or equal to 7.667kohms, between the L-E and the L-N results, then then the 30mA RCD will trip.

The arithmetic I'm using here is: 230 divided by 7667 = 0.030. (Using Ohms Law), and its the difference between the two tests that counts. More than 7.667kA means fix the problem before installing the RCD.

Preparing to be flamed for gross stupidity but would rather make an arse of myself than pretend to know.:blush2::!blank:

 

[bigbaddave]

You figures are correct with regards to when an RCD would trip however anything under 2 meg global and 1 meg individual needs sorting. When I quote for a Db i do IR beetween N&CPC to give me an idear of the condition of the wiring. Anything under 2 global and 1 individualy would indicate that further remedial work will be needed

 

[Sintra]

If your IR results are that low you shouldn't be putting power on to the circuit

 

[acvc]

Thanks guys.

In the event of it being below the permitted minimum I would not go ahead.

Sometimes you get readings way below your meter's max on old wiring but still way over the 'should investigate' threshold. It's this kind of scenario I'm wondering about. So >7.667kOhm difference between these two tests at a level over the permitted minimum would indicate whether an RCD will 'behave'.

 

[telectrix]

any circuit above 2Mohm is considered to be OK, but you would hope to get higher readings than that. don't forget, if you are testing L-N to remove all loads from the circuits. do a soft test first to confirm.

 

[farmerian]

IR no less than 1M ohm anything less than 2M ohms need investigating, RCD test at half X, 1X and 5X, 5X should trip in 40ms with a 30ma rated RCD, but I got to say i don't do much testing now so mite be wrong with my fugues.

 

[bigbaddave]

How do you mean below your meters max, Are you using a Insulation resistance tester ? If so what the range on it?

 

[acvc]

Yeah. What I.m trying (badly) to clarify is e.g. ....... using my Megger 1552 multimeter, I do an IR test L to E and the result is 87.5MOhms, and then do N to E and the result is 75MOhms, then the difference would indicate a leakage current of more than 30mA would be present and the RCD would not behave.

Any difference (not the individual test results) of more than 7.667kOhms between the L-E and the N-E insulation-resistance tests, would cause a 30mA RCD to trip.

 

[telectrix]

so you L-E and you N-E readings are similar. these are way above the minimum values specified. no worries.

 

[bigbaddave]

Those figure are absolutely fine mate.. Theres no chance that there will be causing sufficiant leakage in order to trip a RCD

 

[acvc]

@ Telectrix: it's not the actual figures that I mean. Its the difference between them.

@ bigbaddave: I'm confused. Does this mean that if there is a 30mA leakage from e.g. L to E, and no leakage N to E, then the RCD will still not trip? (230V divided by 7667ohms = 30mA, that's my reasoning).

 

[Sintra]

Forget the earth for now because it isn't needed for the RCD to operate. The RCD operates on an imbalance between L-N only. So even if the difference between your L-E and N-E IR tests is 50 Meg Ohms it will not affect the RCD.

 

[acvc]

Thanks Sintra, but ..... I don't understand? If a L or N went to earth via an exposed conductive part (and therefore an earth) it would trip the RCD wouldn't it? I realise that it the difference between L and N as detected by the RCD an no E is required for the RCD to operate. I'm wondering about whether a leakage from a L or N that would result in more than a 30mA difference between them .....

 

[bigbaddave]

It would yes. But your readings to earth would have to be very low in order to trip an RCD due to earth leakage from bad cable. Dont get to cut up with difference beetween the figures. Its total leakage to earth therefore as your results are way above the acceptable value then an rcd will hold

 

[KAS1]

there would then be an imbalance between L&N

 

[LlandrilloSpark]

I know a guy who has made a little device which houses a 63A 30ma RCD which has two 16MM tails which come out of it, It has the screw terminals cut out of the enclosure so when pricing for a CU change he could disconnect the customers tails from the meter, place them in his device and then the device in the meter, if the RCD tripped he could then pull 3036's and find where the potential headaches could be, the if there was a problem he could warn the customer, the whole process took about 2min and could be carried out with some side cutters and two VDE screwdrivers, along side a nice little sales pitch it worked quite well, His name escapes me....

 

[acvc]

Mmmmm. I'm confused.

The IR test will indicate the level of resistance of the insulation between the bare conductors of cores?
As that resistance is reduced so the current between the two cores will increase while the voltage stays the same?
When that current reaches 30mA will the RCD not trip? Or does the current automatically even up across L and N in this event?

 

[acvc]

Like that big time LlandrilloSpark!

 

[acvc]

Hold on. Would the resistance between adjacent cores in e.g. twin and earth be 'working' in series or parallel. If it's series then the voltage would remain the same but the current would change, if parallel then the other way around.

So maybe its parallel and therefore the current remains the same and that's why the RCD wouldn't trip?

 

[telectrix]

no, because the IR reading is too high to allow either the leakage from the L or the N to get anywhere near the 30mA threshold, then the RCD will not trip. you can have 1Mohm L-E and 100Mohm N-E. as long as each is above the magic 7667 ohms then RCD will not trip.