Is this an error in the "Electrician's Guide to the Building Regulations"

[charlie76]

Reading the "Electricians Guide to the Building Regulations" today (ISBN 978-1-84919-889-9) section 5.7.5 Fixed Equipment, further down the paragraph states:

"Outdoor fixed equipment is not required to be protected by an RCD" That in itself does not ring correct to me I would expect an RCD to be required for all outdoor equipment. It is then followed by "A disconnection time of 0.4s is required for circuits not exceeding 32A". So how is this disconnection time achieved without an RCD?

Is there an error in this wording?

 

[Leesparkykent]

Because your circuit should have a measured earth fault loop impedance less than the maximum permitted.

 

[Andy78]

Reading the "Electricians Guide to the Building Regulations" today (ISBN 978-1-84919-889-9) section 5.7.5 Fixed Equipment, further down the paragraph states:

"Outdoor fixed equipment is not required to be protected by an RCD" That in itself does not ring correct to me I would expect an RCD to be required for all outdoor equipment. It is then followed by "A disconnection time of 0.4s is required for circuits not exceeding 32A". So how is this disconnection time achieved without an RCD?

Is there an error in this wording?

Fixed outdoor equipment in itself may not be required to be protected by RCD, however any associated sockets, wiring methods, or manufacturer's instructions may require this to be present, and it's a good idea too.

Any TN circuit of 32A and under of normal nominal supply voltage is required to disconnect in 0.4 seconds. This is achieved through ADS (automatic disconnection of supply) by keeping the EFLI suitably low to permit this. See table 41.1 and section 411 for further education on the fundamentals.

 

[happysteve]

All sounds fine to me. I think the mention of RCDs is to debunk the myth that just because it's outside, doesn't necessarily mean it needs an RCD. The fact that you thought it might be a mistake in the book justifies its presence.

However, there are plenty of errors in similar publications (the On-Site Guide being of particular note) so it's always worth questioning these things.

Your question about how is a disconnection time of 0.4s possible without an RCD is a little more fundamental. I recommend you look over your college notes on earth fault loop impedance and overcurrent protection devices, and/or chapter 41 of the Regs.

 

[OldeSparky]

example a sub main to a fuse board would not necessarily need to be on a an RCD.

 

[telectrix]

example a sub main to a fuse board would not necessarily need to be on a an RCD.

which has as much relevance to the OP as an ashtray on a motorbike.

 

[OldeSparky]

which has as much relevance to the OP as an ashtray on a motorbike.

why do you say this?

 

[telectrix]

why do you say this?

because i can. :smile5:

 

[Deleted member 26818]

It is quite correct.
The requirement is for mobile equipment used outdoors to be provided with RCD protection.
When you consider that our a.c. supply cycles from 0V to +230V, back to 0V then to -230V and back to -0V fifty times a second (50Hz), then you will see that there are two complete cycles in 0.4secs.
This means that peak voltage (and of course amperage) will occur four times (two positive and two negative), which gives a protective device four occasions where peak amplitude can cause the protective device to operate.
With MCBs/RCBOs, usually peak voltage only has to occur the once for the protective device to operate.
Which is why they have an instantaneous operating time of 0.1sec.
A disconnection time oh less than 0.4secs is easily achievable.

 

[spark 68]

It is quite correct.
When you consider that our a.c. supply cycles from 0V to +230V, back to 0V then to -230V and back to -0V fifty times a second (50Hz), then you will see that there are two complete cycles in 0.4secs.

Out by a factor of 10 there mind Spin , there are 2 full cycles per 40ms or 0.04 secs (hence the significance of the 40ms ms max rule for 30mA RCDs)

 

[Deleted member 26818]

Sorry.

 

[spark 68]

I knew what you were getting at

 

[Deleted member 26818]

Yeah two complete cycles every 0.4secs would be 5 cycles a second or 5Hz.

 

[HandySparks]

...When you consider that our a.c. supply cycles from 0V to +230V, back to 0V then to -230V and back to -0V fifty times a second (50Hz)...

Just so that others aren't misled, a nominal 230V RMS sine wave supply will cycle from 0V to +325V to 0V to -325V to 0V.

The formula is:
Vpeak = VRMS × √2

 

[charlie76]

Thanks guys, some interesting reading and the usual banter. The reason I queried this in the first place was I sat an exam last week and one of the questions was:

1) Which of the following statements is correct for a fixed circuit serving a garden:

The apparent correct answer was:

The circuit must be either PELV or protected by an RCD.

I argued with the examiner that all 4 options were incorrect and he said they were not. I so turns out I subsequently was told that the exam centre apologised and that the question was in fact wrong as it is not a requirement that the circuit is either PELV or protected by an RCD.