Max permissible ZS for rcbo 61009

[atm84]

So, when I did my 2391 I&t course I was told to use to values in table 41.3 of the regs for max ZS value for rcbo which provide earth fault, and over load protection. Recently a colleague of mine said you should use the value in table 41.5 which states a max permissible ZS of 1667 ohms. I don't think that is correct as I didn't think table 41.5 is correct where the device is also providing overload protection.

Any thoughts?

 

[D Skelton]

Ok I can hear your argument but that is contrary to what I was told on the 2391 course and what most other electricians I have spoken to say. However, my colleague agrees with you but states max ZS should be 1667 ohms and not 7667 ohms that you state above.

Why are max ZS values provided for mcbs then if they are irrelevant?

Doesn't matter what you were told on the 2391 course, if you were told different to what I explained then you were told wrong unfortunately.

Max Zs for a 30mA RCD on a TT system is 1667ohms because we use touch voltage (50V) in the equation. On a TN system however we can use the nominal (230V), hence why the max Zs is 7667 on a TN system.

It's not the MCBs that are irrelevant, it's the provision of overload or short circuit protection that's irrelevant. MCBs provide earth fault protection too, that is what the max Zs values are there for.

 

[atm84]

Doesn't matter what you were told on the 2391 course, if you were told different to what I explained then you were told wrong unfortunately.

Max Zs for a 30mA RCD on a TT system is 1667ohms because we use touch voltage (50V) in the equation. On a TN system however we can use the nominal (230V), hence why the max Zs is 7667 on a TN system.

It's not the MCBs that are irrelevant, it's the provision of overload or short circuit protection that's irrelevant. MCBs provide earth fault protection too, that is what the max Zs values are there for.

Ok I think I understand. I think the way I was looking at it was because an RCBO was used then the max ZS should apply to 'MCB' part as this uses a more onerous (lower) ZS value.

From what you are saying, you use the ZS for 'RCD' part. Because as an rcd is used a higher ZS is acceptable to meet th disconnection times.

 

[D Skelton]

Ok I think I understand. I think the way I was looking at it was because an RCBO was used then the max ZS should apply to 'MCB' part as this uses a more onerous (lower) ZS value.

From what you are saying, you use the ZS for 'RCD' part. Because as an rcd is used a higher ZS is acceptable to meet th disconnection times.

I'm not saying a higher Zs is 'acceptable', far from it in fact, but I am illustrating a point that the word 'maximum' means just that. Not 'midway', not 'biggest interpreted value', not 'maximum for one particular component'. But MAXIMUM.

 

[ImpededLoop]

Basically, if you get a fault to earth, you want the device to operate within the required time. With an RCBO, either the mcb or rcd part will operate, so you can base it on the worst case being the rcd part of the device.

Overload or short circuit to neutral are irrelevant where Zs is concerned. Zs only considers a fault to earth. However, if your Ze is good and it's your R1+R2 that is high, you need to consider your R1+RN value to ensure the device will operate under short circuit conditions.