tonights 2391 exam....(10th march 2011)

[lynj]

one thing to say and that is did c&g forget how to be staightforward? felt like some of the questions were half thought......

anyone else take it?

 

[IQ Electrical]

IQ talking to himself again .

BTW the scenario was to install an underground swa using one of the cores as earth to 3 poles for lighting. A switch (can't remember what type) was at the base of the pole and from the switch to the light by flex.

At no time was it mentioned about power to jetty, boats or boat houses. Proximity to water was never declared.

Next time I do a C&G exam I'm going to take a cheeky photo of the paper.

That's my point Paul, and covered by 709.1

 

[Paul.M]

So it does need a 500mA rcd!



Only joking IQ. I put down the test sequence for 30mA like a good boy.

Can't wait to see an actual copy of this paper to check my own answers.

 

[jimmy_one_ball]

So if it was a 500mA RCD then why would the next part of the question, list the sequence of RCD tests, be worth 10 marks when there would only be a couple of stages involved? Thats the part that convinced me to use a 30mA and I seem to remember the wording of it being for additional protection, not fault protection. How did Gra get the wording of the scenario? A photo or memory? I'm desperately trying to hang on to the hope that I might have got more right than I really did !!!!

Also I had the impression the whole installation was RCD protected at the origin, hence the ELFI test method, so there would be RCD discrimination to consider which wouldn't involve a 500mA at the point of utilisation.

I've been sweating over this like mad for the last week, god knows how I'm gonna manage the other 8!

 

[IQ Electrical]

So if it was a 500mA RCD then why would the next part of the question, list the sequence of RCD tests, be worth 10 marks when there would only be a couple of stages involved? Thats the part that convinced me to use a 30mA and I seem to remember it as being for additional protection, not fault protection. How did that guy get the wording of the scenario? A photo or memory? I'm desperately trying to hang on to the hope that I might have got more right than I really did !!!!

Tell me why you would need additional protection on a lamp column?

You would have tests at 1/2 and 1X I delta n on both wave cycles, then the functional button test.

2 marks per answer and 1 for the functional? I can only interpret the regulations applying to the scenario, I gave up guessing City & Guilds marking strategies a few years ago

 

[jimmy_one_ball]

The functional test was the next part of the question and additional protection for small earth leaks? I dunno, thats how I understood the question.

 

[IQ Electrical]

The functional test was the next part of the question and additional protection for small earth leaks? I dunno, thats how I understood the question.

You have to read 709.1 that is the most important regulation in answering this question.

Now that we know that no particular requirements apply unless supplying pleasure craft or houseboats, is there a requirement to provide additional protection to a lamp column on a TT system?

The answer is no.

So we are back to a 100 Ohm value of Ra and an RCD I delta n sized from that value.

That equates to 500mA.

 

[Deleted member 26818]

A 30mA RCD is required for all circuits in TT installations and in special locations.....a Marina is a special location due to the water involved. The scenario deliberately mislead people with the info in the text so some people calculated 500mA. This is irrelevant in a TT system. The question ask you to determine not calculate the RCD tripping current.

Not so.
There is no requirement for TT installations to have RCD protection, it is the preferred method for fault protection, and can be omitted if a low value of Zs can be achieved.

Only circuits of special locations require 30mA RCD protection, and even then, it is only some of them.
In Agricultural Installations for instance the requirement is 30mA for final circuits supplying socket-outlets up to 32A, 100mA for final circuits supplying socket-outlets over 32A and 300mA for all other circuits.

 

[LlandrilloSpark]

Fellas its me again quoting regs!, im done to death with the RCD problem, But i'd like to clear something up from one of the short questions

Question was about the IR test with surge protection devices which COULD NOT be disconnected, Im sure IQ might have a condradicting reg number for me but id like to point you to reg 612.3.2 the paragraph below table 61 which says reduce the IR test voltage to 250 VDC and minimum acceptable value of 1 MEG OHM

 

[malcolmsanford]

Fellas its me again quoting regs!, im done to death with the RCD problem, But i'd like to clear something up from one of the short questions

Question was about the IR test with surge protection devices which COULD NOT be disconnected, Im sure IQ might have a condradicting reg number for me but id like to point you to reg 612.3.2 the paragraph below table 61 which says reduce the IR test voltage to 250 VDC and minimum acceptable value of 1 MEG OHM

Not sure what reg IQ quoted as this thread is 15 pages long, but I would lay money on the fact he quoted the right one ...........can you remember what he did say

 

[IQ Electrical]

Fellas its me again quoting regs!, im done to death with the RCD problem, But i'd like to clear something up from one of the short questions

Question was about the IR test with surge protection devices which COULD NOT be disconnected, Im sure IQ might have a condradicting reg number for me but id like to point you to reg 612.3.2 the paragraph below table 61 which says reduce the IR test voltage to 250 VDC and minimum acceptable value of 1 MEG OHM

No objections from me

I don't know what that question asked exactly but that is acceptable for testing with in-circuit surge protection devices.

I think I posted exactly that somewhere in this small thread!

 

[IQ Electrical]

Post 154

 

[malcolmsanford]

You disconnect a surge protective device or you conduct the IR test at 250V DC but with 1 MOhm still used as a minimum value BS7671:2008 page 158.

The disconnection time on a BS EN 61008/9 RCD at 1X I delta n is 300ms. I would bet large sums that C & G would not use a 'BS' residual current device in this exam.

I can't comment on the rest without the paper as there are too many variations on the important parts of the questions.

Post 154

I don't know who is sadder me trying to find post #154 or you KNOWING it was post #154.

I think IQ quoted reg 612.3.2 pretty well but he did leave out the part about other equipment ..................

 

[IQ Electrical]

I don't know who is sadder me trying to find post #154 or you KNOWING it was post #154.

I think IQ quoted reg 612.3.2 pretty well but he did leave out the part about other equipment ..................

Thanks Malcolm!

The note on testing with in-circuit surge protective devices is one of my suggestions for inclusion in the amended Guidance Note 3-there is no mention of it in the existing book!

 

[B6 7BU]

then the last part of the Q quoted the max disconnection time of a TT as 0.2s so I said the BSEN should be replaced with a BS to match the 200ms max permitted disconnection time. Well you don't have to get them all right do you lol

To meet 200ms requirement we need to test at 2 x IΔn (4 marks)

 

[LlandrilloSpark]

I posted a reg about the Rcd and additional protection for special locations, IQ found a reg which changed things, I wasn't having a go