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]

So what is the Zs and the type C 20A device for the circuit telling you.

 

[LBSparky]

So what is the Zs and the type C 20A device for the circuit telling you.

I'm not sure. According to table 41.3 the Max Zs value for a 20a type C breaker is 1.09a but I don't understand how that helps me answer the question.

 

[westward10]

Understanding what they are giving you will help you understand the answer to the question. Regulation 411.4.204 states where an RCD is used to satisfy the other quoted Regulations this should tell you that without the RCD the the other quoted Regulations are not satisfied, the Zs given and the type C, 20A device should tell you this. You say Table 41.3 gives a maximum value of 1.09 but you have been given 1.69 so what are the possible consequences should a fault occur.

 

[LBSparky]

Understanding what they are giving you will help you understand the answer to the question. Regulation 411.4.204 states where an RCD is used to satisfy the other quoted Regulations this should tell you that without the RCD the the other quoted Regulations are not satisfied, the Zs given and the type C, 20A device should tell you this. You say Table 41.3 gives a maximum value of 1.09 but you have been given 1.69 so what are the possible consequences should a fault occur.

One consequence would the protective will not operate in the required time (0.4 seconds)

 

[westward10]

One consequence would the protective will not operate in the required time (0.4 seconds)

Essentially yes so the quoted Regulations in 411.2.204 are not satisfied (fault protection not satisfied) but an RCD can be used to satisfy them and this is confirmed in Regulation 411.4.5 now the question is which rating of RCD to use. The question refers to it being a ring final circuit likely to incorporate socket outlets however it does not cite this so this could be deliberate. Would any or all of the RCDs in Table 41.5 satisfy fault protection with a Zs of 1.69.

 

[LBSparky]

Essentially yes so the quoted Regulations in 411.2.204 are not satisfied (fault protection not satisfied) but an RCD can be used to satisfy them and this is confirmed in Regulation 411.4.5 now the question is which rating of RCD to use. The question refers to it being a ring final circuit likely to incorporate socket outlets however it does not cite this so this could be deliberate. Would any or all of the RCDs in Table 41.5 satisfy fault protection with

If you use the formula stated in 411.5.3 ( Ra x IΔn < 50 ) then none of them comply. 1.69 x 30 = 50.7ohms

 

[Risteard]

If you use the formula stated in 411.5.3 ( Ra x IΔn < 50 ) then none of them comply. 1.69 x 30 = 50.7ohms

You should be multiplying by 0.03 (Amperes).

I presume you meant Volts rather than Ohms.

 

[LBSparky]

You should be multiplying by 0.03 (Amperes).

I presume you meant Volts rather than Ohms.

In that case of all them comply. Would the answer be a 30mA RCD then?

 

[westward10]

If you use the formula stated in 411.5.3 ( Ra x IΔn < 50 ) then none of them comply. 1.69 x 30 = 50.7ohms

Do you know the earthing system because that is in reference to a TT system.

 

[LBSparky]

Do you know the earthing system because that is in reference to a TT system.

It's TN-C-S system. If not by using table 41.5 then how do you work this out? Reg 411.4.204 specifically mentions table 41.5

 

[westward10]

Yes it does but ignore that equation above it is for TT systems.
Table 41.5 has devices ranging from 30 to 500ma with maximum Zs values ranging from 1667 to 100 ohm these all far exceed your 1.69 so what does this tell you.

 

[LBSparky]

Yes it does but ignore that equation above it is for TT systems.
Table 41.5 has devices ranging from 30 to 500ma with maximum Zs values ranging from 1667 to 100 ohm these all far exceed your 1.69 so what does this tell you.

That all of them are compliant?

 

[westward10]

That all of them are compliant?

Well yes in terms of providing fault protection. What you need to decide is whether the circuit needs additional rcd protection on top of fault protection.

 

[LBSparky]

Well yes in terms of providing fault protection. What you need to decide is whether the circuit needs additional rcd protection on top of fault protection

The question isn't asking if the circuit needs RCD protection. It's asking what RCD rating is needed to comply. Can someone please just explain to me how to work out what rating is required?

 

[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.