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

 

[telectrix]

at my age, i won't be taking the 18th, but if it was anything like 17th, i'd finished in 50 minutes, scored 87%, and that was at the age of 65. i could have done a bit better if i'd taken the time to look a couple of things up, but the pub was open.

 

[westward10]

Yea that is interesting,20 amp?

You can get an rcd shunt trip that can be set for a range of currents up into the amps.

 

[telectrix]

You can get an rcd shunt trip that can be set for a range of currents up into the amps.

never seen one of them.

 

[cliffed]

at my age, i won't be taking the 18th, but if it was anything like 17th, i'd finished in 50 minutes, scored 87%, and that was at the age of 65. i could have done a bit better if i'd taken the time to look a couple of things up, but the pub was open.

Yea this will be the last I do,done 14,15,16,17 th,this was the most difficultist,its the new regs that disturb you.

 

[Derrick Winn]

Well I am th same started in 1971 14th edition did not do anything else until part P came in, and then it's been nothing but pay out for courses books schemes . But now at nearly 63 decided to quit. Have had enough

 

[Deleted member 26818]

Not really sure what that question is about?
The maximum with an Ra of 2 ohms would be 25A, so all 3 of the answers shown, can be used.
Not aware of there being a limit of 100mA for TT installations.
Though I seem to recal something about 100mA max for fault protection and 300mA for fire protection?
However the question doesn’t mention fault or fire protection.

 

[cliffed]

Well I am th same started in 1971 14th edition did not do anything else until part P came in, and then it's been nothing but pay out for courses books schemes . But now at nearly 63 decided to quit. Have had enough

Yea agree same age,finishing in 2 years

 

[DefyG]

Never seen a RCD (61008) with trip current (I delta n) of 20A.
Do anyone make one?

 

[TJ Anderson]

Never seen a RCD (61008) with trip current (I delta n) of 20A.
Do anyone make one?

Me neither. 60898 C2 would clear a 20A earth fault in 0.1s but you wouldn't have much capacity lol. Don't even know why I wrote this haha

 

[westward10]

You can get earth leakage shunt relays which sense leakage by way of core balanced current transformers. These can be set for milliamps or amps but are not your run of the mill devices.

 

[Nigel]

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.

Given the options I would say 300mA as that is the closest option to 20A.

However there are four options and you have only given us three.

 

[Ffoeguk]

Table 53.1 page 156
Gives the answer to the question