18th edition question

[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