How to determine the RCD rating in order to comply with 411.4.204

[LBSparky]

Hey guys. I am currently studying for 2396 city and guild electrical design exam and I'm stuck on this question. Its asks to determine the residual current rating of an RCD in order to comply with regulation 411.4.204. The circuit in question is a final ring circuit protected by a 20a type C circuit breaker, with a Zs value of 1.69 ohms. Looking at table 41.5, is a 500a RCD required since the Zs value does not exceed 100oms? Thanks for all the help.

 

[westward10]

Can you show the actual question.

 

[LBSparky]

Can you show the actual question.

An RCD has been installed in a single phase circuit due to the measured Zs exceeding the permitted value for the installed circuit breaker.

The circuit is wired using thermoplastic 70c flat profile twin and cpc cable with 2.5mm live conductors and 1.5mm cpc to a length of 58m

The measured Ze at the origin of the circuit and TN-C-S installation is 0.33ohms. The circuit is protected by a 20A type C circuit breaker to BS EN 60898.

Determine the residual current rating of an RCD in order to comply with Regulation 411.4.204.

 

[Risteard]

An RCD has been installed in a single phase circuit due to the measured Zs exceeding the permitted value for the installed circuit breaker.

The circuit is wired using thermoplastic 70c flat profile twin and cpc cable with 2.5mm live conductors and 1.5mm cpc to a length of 58m

The measured Ze at the origin of the circuit and TN-C-S installation is 0.33ohms. The circuit is protected by a 20A type C circuit breaker to BS EN 60898.

Determine the residual current rating of an RCD in order to comply with Regulation 411.4.204.

How did you ascertain that this was a ring final circuit then? If it was then it would be supplying socket-outlets, and would require additional protection. I don't see any of that mentioned in the question.

 

[LBSparky]

How did you ascertain that this was a ring final circuit then? If it was then it would be supplying socket-outlets, and would require additional protection. I don't see any of that mentioned in the question.

That was just an assumption I made. Again, the question is NOT asking whether the circuit needs additional protection. The question has already determined that the circuit requires RCD protection. I need to know how to determine what RCD rating is needed to comply with regulation 411.4.204

 

[westward10]

Theoretically any of those RCDs in the Table would satisfy however 300ma and above are often used for fire protection and selectively from downstream devices so this leaves you with 30 or 100ma. The question is scant in detail so you have no idea whether additional (30ma) protection is required so as a minimum make a decision.

 

[westward10]

How did you ascertain that this was a ring final circuit then? If it was then it would be supplying socket-outlets, and would require additional protection. I don't see any of that mentioned in the question.

Not necessarily it could just supply fixed equipment. The question I believe is deliberately scant on detail to concentrate on a rudimentary answer because if it stated socket outlets the student would automatically select 30ma.