18th edition question

[cliffed]

Had these on the 18 th exam,confused me a bit.
What’s the maximum Rcd that can be used ,when an Earth electrode Reading is 2 ohms.
100 ma
300 ma
30 ma.
What would you say is the answer.

 

[telectrix]

deleted

 

[telectrix]

with a fault current of 115A, what do you think will trip i 200mS or less?

 

[cliffed]

Yea I know,told we didn’t need calculators,but putting that aside,what logical answer would it be.

 

[cliffed]

with a fault current of 115A, what do you think will trip i 200mS or less?

I’m asking,I’ve already ticked the answer,so just wondering logically,what do you reckon.

 

[telectrix]

300mA , but somewhere I recall BS7671 or a GN recommending 100mA for a TT installation ( presumably to allow for variations in the Ra)

 

[westward10]

Is that actually a genuine 18th Ed question or something from online.

 

[Loki]

Could they be relating to this table on page 64

but from memory i thought you used a 100mA RCD on TT & then 30mA at board.

 

[Loki]

Also found this on page 156

 

[telectrix]

now that is interesting. the bit about insulated and sheathed cables meeting the requirements of double or reinforced insulation.

 

[cliffed]

Is that actually a genuine 18th Ed question or something from online.

Yea,I actually ticked the 300 ma,being the max,obviously the ideal would be the 30ma.

 

[cliffed]

Also found this on page 156

View attachment 45943

Yea that is interesting,20 amp?

 

[telectrix]

Yea,I actually ticked the 300 ma,being the max,obviously the ideal would be the 30ma.

nope. you'd have 30mA for final circuits. if you had a 30mA for fault protection, that would contravene the division of circuits and discrimination of RCDs in series.

 

[cliffed]

Passed the exam,thought it was relatively hard,& you will need the 2hrs.

 

[cliffed]

nope. you'd have 30mA for final circuits. if you had a 30mA for fault protection, that would contravene the division of circuits and RCDs in series.

Yea ,agree just panicked I think,it’s the max,then churned me up