RCD trip times anomilies

[fiardor]

Hey guys i have a question.

A 63A 30ma RCD main switch protecting a 20 metre run of FP200 16A mcb in a 2 way ccu.

Would you expect the RCD test results to be the same at source as you would at the end of line of the FP200?

My NIC qualified electrical supervisor basically says that he takes his reading at E.O.L and then writes his readings down on page 3 for 1x and 5x for that circuit.
He then writes the same value in the 1x for the 63A RCD main switch on page 2.

He says that the RCD will trip at the same time regardless of the circuits length or characteristics.

Is this true?

Surely its the circuit that will determine the trip time not the RCD.

This guy is starting to annoy me with many aspects of testing and form filling but as he signs off my test sheets should i just acquiesce or kick his head in (joke)

I am 2391 and 17th JIB approved but until recently i was 100% industrial and commercial and have never seen test sheets like these before and i am finding many discrepencies from what i am used to.

Many thanks for taking the time to reply in advance.

Fiardor

 

[Octopus]

Did you test the circuit EOL and the RCD at source? If so what were the readings?

 

[Deleted member 26818]

Overcurrent devices operate when a pre-set temperature is achieved.
This temperature is caused by current flow through the device. The greater the temperature, the faster the device will operate.
Current flow in turn is affected by resistance. The greater the resistance the lower the current flow.
As such if a fault occurs on a circuit with high resistance, the device will take longer to operate than an identical device on a circuit with low resistance.
We determine operating time as being the interval between when a fault occurs and when the device operates. This in affect is how long it takes the temperature to rise to reach the device's pre-set temperature.
RCDs on the other hand detect current imbalances between live conductors. They operate when a pre-set imbalance is achieved.
Operating times for RCDs are determined not from when a fault occurs causing an imbalance, but from when the imbalance is such that it meets the rating of the RCD.
As such circuit characteristics will have no effect on RCD operating times. The characteristics may affect how long it takes a fault to cause an imbance which would then cause an RCD to operate, however that has no bearing on the RCD operating time.
Circuit characteristics will however affect RCD testing, depending on the method and type of tester used.
Most tester operate by injecting current into the neutral conductor thereby causing an imbalance that the RCD detects.
If the resistance between the RCD and where the test is conducted is too great, it will cause the injected current to fall below an amount that the RCD will detect.

 

[topquark]

You should test the RCD at the CU and record the results on the line for the RCD from that. If you wish to be thorough you would also test from the accessory with the highest Zs (for each of the circuits protected by that RCD (or RCBO for that matter)). With such a short run of cable the results should be virtually identical (I say virtually because you seldom get exactly the same time on a repeat test of a device).

The requirement in BS7671 is to test the RCD itself. Testing the Zs for each circuit should be sufficient to prove that the RCD should operate in time for that circuit.

 

[fiardor]

Okay guys many thanks, very helpful and inciteful and i shall not mention this to my supervisor (head bowed down)

Thanks again.

Fiardor

 

[nickblake]

i always test the RCD at the end of the circuit , do the Zs then the RCD test , one of the reasons is if theres going to be a fault its going to be some where along the circuit not at the RCD so i like to be sure its going to operate where there's going to be a likley fault ,and if it will trip at the circuit end it will trip at the board

 

[Octopus]

Testing the Zs and then the RCD on each circuit, at the furthest point, is the way I do it, sure it takes a bit longer but I want to be able to sleep at night.

 

[IQ Electrical]

The requirement is to test the RCD regardless of what it is supplying.

The circuit cable and anything that the circuit is supplying can have capacitive coupling which although makes no difference to the actual RCD disconnection time, may distort the time that your tester actually displays.

That is the reason for testing at the terminals of the device with all load disconnected.

 

[telectrix]

i generally test at a socket outlet. if i get a reading that is outside the accepted, then i will test again at the RCD before condemming it.