Confused...max electrode resistance with RCD?

[DaveS]

Sorry guys. Probably sick of me by now got exam tmrw and checked old posts and books but still bit confused with allowable max electrode values. Does max 200 ohms still apply if RCD is inplace? If so why do we even have a max resistance for rcd disconnection if these values are so much more than 200? Cheers

 

[telectrix]

you're getting confused. a Ra value as low as possible with a TT system is desirable. BS7671 suggests that a value of >200 ohms may not be stable, but it does not stipulate a max. value. a 30mA RCD will disconnect with values up to 1667, but that's more like a get out of jail free card for not getting an acceptable Ra/Ze.

 

[TJ Anderson]

Sorry guys. Probably sick of me by now got exam tmrw and checked old posts and books but still bit confused with allowable max electrode values. Does max 200 ohms still apply if RCD is inplace? If so why do we even have a max resistance for rcd disconnection if these values are so much more than 200? Cheers

Can I ask, do you think 200 ohms (Zs) would provide satisfactory disconnection without an RCD?

 

[DaveS]

Cheers Telectrix so this does infact mean that with the use of RCD even with a high Ra/ze we are still complying with all required regs if under say the 1667 of RCD 30ma

 

[telectrix]

Can I ask, do you think 200 ohms (Zs) would provide satisfactory disconnection without an RCD?

a dead short would give a fault current od 230V/200ohms . i.e. 1.15A. how's that going to blow a 100A fuse?

 

[DaveS]

Can I ask, do you think 200 ohms (Zs) would provide satisfactory disconnection without an RCD?

Not too sure to be honest TJ everything I've read seems to suggest less than 220 for total zs. 200 for electrode?

 

[telectrix]

Cheers Telectrix so this does infact mean that with the use of RCD even with a high Ra/ze we are still complying with all required regs if under say the 1667 of RCD 30ma

the problem is with TT systems that if you c an't get a Ra/Ze of <1 ohm, then you are relying on the RCD for fault protection, so the 1667 figure is the limit.

 

[TJ Anderson]

a dead short would give a fault current od 230V/200ohms . i.e. 1.15A. how's that going to blow a 100A fuse?

Exactly, I was trying to get OP to think about it

 

[DaveS]

Cheers guys I get the equation and why the use of RCD protection and values used I think (head pretty scrambled been at it all night)

 

[TJ Anderson]

What exam have you got tomorrow?

 

[DaveS]

2394/95 TJ only multiple choice tmrw but not taking it for granted believe me

 

[telectrix]

take the time to read the questions properly and make sure you know what they want.

 

[DaveS]

take the time to read the questions properly and make sure you know what they want.

Thanks tel. I've put quite a few hours into it hopefully be ok although going through some papers tonight did misread quite a few things will watch out for that. Early night tonight for me I think

 

[TJ Anderson]

All the best for tomorrow.

 

[DaveS]

All the best for tomorrow.

Cheers TJ

 

[Leesparkykent]

Good luck DaveS.

 

[DaveS]

Good luck DaveS.

Cheers Lee thanks mate

 

[Richard Burns]

Cool calm and collected.
read the question.
answer only the question.

Good luck, I am sure it will be a breeze!

 

[DaveS]

Cool calm and collected.
read the question.
answer only the question.

Good luck, I am sure it will be a breeze!

Thanks Richard advice appreciated mate

 

[malcolmsanford]

Good luck for the exam. Just remember cardinal rules

• Multiple choice read them all first and do the ones you know first. Don't spend too much time on ones your not sure about and then run out of time
• Read the question, and really read it, the C&G love wording questions that can be ambiguous
• Terminology it is not a Meggar it is an insulation resistance, it's a ring final circuit, not a ring main, this is vital and so easy to get wrong.
• Try and chill, don't panic and just think things over.

And good luck