Please Help a student !

[acestu]

Hi could anyone help me to do this calculation and explain it please,

Calculate the size of Main rcd for automatic dissconection needed for a domestic TT instalation with an earth electrode resistance of 150 ohms



thanks in advance
acestu

 

[Mac]

What type of circuit are we talking about mate

 

[acestu]

Sorry Mac I have edited it now , it's the main RCD

thanks
acestu

 

[not clever]

What you are probably wanting is the touch voltage...not to exceed 50v

So for a 100amp 30mA rcd

Ze x I delta n / 1000

150 x 30 = 4500

4500/1000 = 4.5v

 

[Mac]

Hi could anyone help me to do this calculation and explain it please,

Calculate the size of Main rcd for automatic dissconection needed for a domestic TT instalation with an earth electrode resistance of 150 ohms



thanks in advance
acestu

The main fuse at the CU is normally 100A.All the circuits on a TT domestic system should be RCD protected anyway.Maximum disconnection time on circuits up to 32A is 0.2 secs.Maximum disconnection time for circuits exceeding 32A is 1sec.Its all in the regs,Chapter 4 i think.Reg 410.3.3 relates to ADS

 

[acestu]

Thanks not clever, the exact wording of the question is:

State the operating current for a residual current device (RCD) used to provide:

a.) protection against fire risk in a farm building

b.) additional protection for equipment outdoors

c.) automatic dissconection on a domestic TT instalation having an earth electrode resitance of 150 ohms



I know the formula has the 50 volts in it somewhere like you said, and the formula

RA x Delta n less than or equal to 50 volts is what i found in the regs

So in this question c. I have the info 150ohms , 230Volts, and 50 volts

just need to know which way to do it to get the miliamp rating of the RCD


Hope that is clear

Thanks Again
Acestu

 

[scotsparky]

a/ fire protection only 500ma

b / Externatial fult protection 30ma

c / 300ma 150 x 300 /1000 is 45 v so maximum is 300macd

 

[Mac]

Thanks not clever, the exact wording of the question is:

State the operating current for a residual current device (RCD) used to provide:

a.) protection against fire risk in a farm building

b.) additional protection for equipment outdoors

c.) automatic dissconection on a domestic TT instalation having an earth electrode resitance of 150 ohms



I know the formula has the 50 volts in it somewhere like you said, and the formula

RA x Delta n less than or equal to 50 volts is what i found in the regs

So in this question c. I have the info 150ohms , 230Volts, and 50 volts

just need to know which way to do it to get the miliamp rating of the RCD


Hope that is clear

Thanks Again
Acestu

Ohms Law mate.230V / 150Ohms / 1000=0.00153mA

 

[acestu]

Thanks Scotsparky, so i take it that the answer for c. should be: anything up to 300ma and not just 300ma, the wording of the questions does my head in


cheers
Acestu

 

[MrSkipsey]

You wait till the exams. My dog writes more cohearent then C&G!

 

[Guest123]

Hi.

Question A is a bit of a wrong-un as it kind of contradicts itself because all circuits other than socket outlets (30mA) in an agricultural installation must be protected by a 300mA RCD. Section 704 ol' red.

More sneaky than a sneaky thing in sneaky town with an NVQ 3 in sneakyness!!!!!!

 

[scotsparky]

so cunning you could put a tail on it and call it a weasil.....................Long live Baldrick from blackadder.