2391-01 question

[tighec2]

Hey guys,

i'm just studying for the 2391 theory next thurs and have a question:

State three reasons for the use of a 20mA RCD

i'm not entirely sure what the answer to this but i'm thinking:

1. If you can't guarantee earth electrode resistance of less than 200ohms in a tt instalation
2. in special locations such farms / construction sites with 25V max touch voltage
3. in special locations such as caravan

if anyone can agree or disagree with these please let me know!!!

Cheers...

 

[electricAl]

I think you`ll find this is a very old question. Did you get it here by any chance?

This examination consists of two sections

Trouble with many of the `freely` available past papers - they`re very past, you won`t get asked them, & can send you off on tangents when you could get a better return by just studying GN3 itself.

I`m sure if you asked, someone on here would help you source some more recent questions - have you done a search for 2391 papers ?

 

[iceman]

when workin out the if the circuit conductor is big enough to cape with the on 40A circuit breaker wot diconnection time do i enter into the equation example
/ I2t
s = ------
k

/435 x 0.1 this is wot i mean by disconnection time, is this rite for a 40A type B circuit breaker?
------------
115

= 1.2mm

any help would be great cheers

 

[securityvillage]

Spot on!

GN3 and exam success are a must. The people who set the examinations have written the exam guide, it tells you exactly what they are looking for when answering questions.

My 2391 exam on Thursday too. Will be cramming like mad at the weekend.

 

[iceman]

has for disconnection times for 32A and 6A circuit breakers do i enter 0.4?

 

[electricAl]

has for disconnection times for 32A and 6A circuit breakers do i enter 0.4?

Yes mate. Upto & including 32A = 400ms

"Greater than 32A" = 5 secs

As to your earlier Q - it`s a bit unclear what you`re asking Iceman. Are you using the adiabatic in appendix of GN3?

 

[iceman]

yes im usin the adiabatic equation im just abit unsure about the 0.1 time for the 40A circuit breaker how does come up with 0.1?

 

[zupos40]

yes im usin the adiabatic equation im just abit unsure about the 0.1 time for the 40A circuit breaker how does come up with 0.1?

what was the full question

 

[electricAl]

Where did you get the 0.1 time from. Are you working on a given sample question/answer

Type b`s are rated to pop at x5 upto about 10 secs so don`t matter a really whether it`s 0.1 or 0.4 cos they`ll be the same...a 40a will `need` a fault current of 200a to be guaranteed to trip (40a x 5)

Confused as to where you got the 435 from?

Basically, now you`ve aquired your max fault current (I) you square it then multiply by time (as above) then find the root of that sum & divide by your k factor for chosen cable (you`ve used 115 as for regular thermoplastic as per t&e)

Come back if your still unsure mate

 

[zupos40]

Where did you get the 0.1 time from. Are you working on a given sample question/answer

Type b`s are rated to pop at x5 upto about 10 secs so don`t matter a really whether it`s 0.1 or 0.4 cos they`ll be the same...a 40a will `need` a fault current of 200a to be guaranteed to trip (40a x 5)

Confused as to where you got the 435 from?

Basically, now you`ve aquired your max fault current (I) you square it then multiply by time (as above) then find the root of that sum & divide by your k factor for chosen cable (you`ve used 115 as for regular thermoplastic as per t&e)

Come back if your still unsure mate

I assume that the value of I = 435A was obtain from Uo/Zs design or measure, so if you the goto Appendix 3 fig 3.4 for 40A type B mcb the disconnection time would be less than 0.1 sec (theorically) even a 40A type C would disconnect in less than 0.1 sec.

I believe this is where 435 and 0.1 comes from.

 

[iceman]

thnks for ya help guys i got the calc from scaddens design and verification based on the new regs

 

[electricAl]

Makes sense.

Would necessitate you knowing the Zs value tho

I can only assume that was given in the question (.53 then)

You`re obviously familiar with this or similiar questions - is it in the revision book that so many use?

 

[iceman]

yeah its a great book for workin out design calcs iv leaned loads in there this week i just hope iv done enough for the 2391 next week

 

[tighec2]

I think you`ll find this is a very old question. Did you get it here by any chance?

This examination consists of two sections

Trouble with many of the `freely` available past papers - they`re very past, you won`t get asked them, & can send you off on tangents when you could get a better return by just studying GN3 itself.

I`m sure if you asked, someone on here would help you source some more recent questions - have you done a search for 2391 papers ?

yes mate thats where i found it, i had realised that this paper was old trying to use it for relevant parts...i was actually thinking after that 20mA scenario mightn't be a requirement of the regs anymore