RCD question

[bs7671]

Hi people can anyone tell me the difference between the tripping currents and the current ratings of rcds.Any help I would be thankful of

 

[antimatter]

30 mA Normally trips @ around 24 - 27 mA

 

[malcolmsanford]

Tripping currents are the Ma rating you see on the RCD ie a 63a 30Ma. (Milli-amps)

This means under fault conditions the RCD will trip with only a 30Ma fault. As a fault can develop many 100's of amps very quickly under fault conditions you can see that 0.003amps is a very small amount and therefore will trip quickly.

Which is why when we test a RCD the normal tests are carried out in Ms (Milli-seconds) to see the trip times rather than the amount of Ma taken. There is a Ma test called a "ramp" test for Ma but that is another story.

The current rating is the 63a 30Ma part.

This is the rating for the circuit or cable load in amps. The circuit design and cable loading which is worked out in the design stage. So an example is a lighting cicuit in your home is most liklely 6 or 10 amps. That takes into account the number of lights which is the load and therefore the cable used.

The 6 or 10 amps protects the cable from damage. So you lighting RCD would be 6a or 10a 30Ma

 

[antimatter]

Sorry, i think you mean 0.03 amp instead of 0.003 amps.

 

[malcolmsanford]

ooopps fat fingers small key board, yes 0,03 thank you must try reading the reply next time, which I did and still missed it

 

[bs7671]

Many thanks Malcomsanford brilliant answer.

 

[malcolmsanford]

Many thanks Malcomsanford brilliant answer.

Most welcome sorrt about the 0.003 slip though lol

 

[bs7671]

I do have another question regarding r1+r2 testing.Would cross connecting line to cpc prove there are no parrellel paths and no earth faults.How exactly is it done?

 

[antimatter]

It would not prove there are no parallel paths, it proves continuity of the protective conductor and the line conductor, and polarity. The circuit being tested has the CPC and line conductor connected together (de-energised first) and futherest point / highest reading gives you your R1 + R2 for the circuit.

Parallel paths are from incoming sevices (gas, water, et al) lowering the overall Ze, thats why these protective conductors are to be removed (de-energised first) before a Ze test.

 

[malcolmsanford]

As this will become part of your course then the R1 + R2 will be done as a precursor to getting your Zs, as in Zs = Ze + ( R1 + R2).

In the real world the 17th edition meters will give you the Zs on a final circuit, etc by measurement. The measurement results will invariably be lower than if you did the test by calculation as that is when you get those parallel paths. Most electricians do the measurement method, though the NICEIC advise that Zs should be done by calculation as it's a live test, but that is another thing

If you left the Earth connected to the earth bar and then did the loop method you would get parrellel paths, off the commoned earths from other circuits and all your bonding. On new installations a lot of testing for initial verification is done before the connection to the CU as you will want to prove polarity at this point. On a PIR it can be slighlty different.

So with earth connected to Earth bar parallel earths, disconnected no parrallel earths

Only an insulation resistance test will pick up a fault, the cross over test will prove polarity

 

[antimatter]

Remember that polarity is confirmed first in your "dead tests" CPC CRFC, and then again when the installation is energised to confirm the polarity is correct on the incoming supply.

 

[bs7671]

You are a wealth of electrical knowledge!I am trying to get my head around some of this and have found this forum one of the best resources.Educated people are willing to help us newbies by sharing their understanding and I think its great.I have got hundreds of questions and one day hope to be in your shoes sharing my expert advise.Thanks very much again

 

[bs7671]

How do you carry out live testing of a single phase rcd?

 

[antimatter]

No problems buddy, glad to help, press the thanks button if you think we deserve it!!

 

[antimatter]

On any circuit that is part of the RCD, plug in the RCD tester and test @ 1/2 times there should be no trip within 2 seconds, test again @ 1 x and the RCD should trip within 200ms and again on the 5x setting (additional protection) the RCD should then trip within 40ms. Then press the test button on the RCD lastly.
This test should but never is repeated by the owner of the installation every three months.

All these tests should be conducted @ 0 and 180 degrees setting of the RCD tester and the highest figure recorded.

(All other tests should be exectued first as RCD testing is one of the last tests to be conducted on circuits).