Trainees! Pop Quiz: Outside Light

[happysteve]

Another pop quiz. Either just take a pop at the answers using common sense, or post reg numbers if relevant/if you can.

You're asked to install an outside floodlight on the side of a customer's bungalow in England, the sort that comes with an integral PIR sensor. Earthing system is TN-C-S; BS3036 semi-enclosed fuses, no RCD(s), earthing and bonding all good. It's relatively easy to take a feed off the existing lighting circuit (5A) in the loft, drilling through the exterior wall.

(1) Is the new light within the scope of section 714 of BS7671 ("Outdoor lighting installations")? In other words, is it a "special installation or location" (according to BS7671)?

(2) Is this work notifiable to Building Control? ( https://www.gov.uk/government/publications/electrical-safety-approved-document-p )

(3) Is it necessary to provide a double pole isolator?

(4) Is it necessary to fuse down the new part of the circuit to the light?

(5) Is it necessary to provide RCD protection for this outside light?

(6) How, in practice, would you do the job?

(7) If the property was in Wales, would it make a difference to any of your answers (and if so, under what circumstances)?

Answer a part, or all - don't be shy, have a go!

 

[Karin]

okay, think i've compiled an attempt at everything now
cheers again for this steve. interesting to see if i'm thinking in the right places!

1.
no, BS 7671 714.1(v)

2.
no, Approved Document P, 2.7

3.
TN-C-S system so double pole isolation unneccessary. 537.1.2
i can't find anything to suggest that this light would need it's own isolator any more than the rest of the lights on the circuit, so the 3036 fuse(?, not listed in table 53.4) or main switch would do

4.
if the current carrying capacity of the cable used to install the light is less than 5A then fusing down would be required (BS7671 433.2.1)
if the current carrying capacity is the same, or does not decrease below 5A then no fusing is neccessary and 433.3.1(i) may apply.

5.
looking in section 522 i'm leaning towards no for any external influence. for impact, if the cable passes near enough horizontally and is obvious from both sides, then perhaps it is not "concealed in the wall" as such and would not require it?

6.
a bit of tough black rubber flex from the light into a junction box, preferably in the loft. some cable clips. if i'm wrong about not requiring RCD protection due to impact then i'd use an offcut of galv conduit to avoid that problem

7.
i'd think no, as security lighting is specifically mentioned as non-notifiable when fixed to an external wall of the house, though there is this mention of "exposed outdoor connections" that would make it notifiable. it seems to be obvious that you wouldn't leave exposed connections though?! even if i used a junction box outside the house it'd be IP65, so it can't mean that?
(note i. to Table 2, Approved Document P for Wales)

 

[hightower]

I'm in agreement with Karin's answers - they all seem sensible and I was thinking along the same lines in all questions.

 

[Richard Burns]

Good answer there I think.
re: number 4 have a look at table 52.3 and the associated current carrying capacities of permitted cables.

 

[Karin]

Good answer there I think.
re: number 4 have a look at table 52.3 and the associated current carrying capacities of permitted cables.

thanks Richard.
i see Table 52.3 suggests minimum permitted size TnE is 1,0mm² which will carry 5A all day long, and minimum flex is 0,75mm² which has a CCC of 6A before any derating so would be fine anyways.
there is also reference to specific appliances being allowed minimum CSA as specified in the product standard. does this mean that if the MI suggested a minimum of 0,5mm² flex to be used you could connect into the radial with that through a 3A fuse? or is that just for removable applicances connected with a plug and socket?

 

[happysteve]

In answer to your most recent post... I would generally steer clear of anything less than 0.75mm flex...

As for the other answers - very good! Spot on! As for your query about "exposed outdoor connections" - I'm not sure what that means either.

I guess what I wanted folk to take away from this is, there are some things that are myths, and other things that you do because they are good practice. The intention was that you don't take some guy on the internet's word for it, you get used to the idea of looking stuff up to convince yourself.

Ok, so now we've worked out that an RCD is not necessary for an outside light (unless other issues such as cables buried <50mm etc), and nor is a double pole isolator/switch, let's consider a slightly different scenario:

TN-C-S, this time with a dual-RCD split board (so all circuits are protected by one of two 30mA RCDs). A feed off the lighting circuit as before (6A type B circuit breaker this time), and we have correctly determined that we don't need to fuse it down, as the current carrying capacity of our cable will be sufficient.

We don't need a DP isolator/switch - but why might we fit one?

 

[Karin]

to easily isolate the fitting from the rest of the house in case of neutral-earth fault tripping all circuits on the RCD? because an outdoor fitting is more likely to be suceptible to eventual water (or insect, etc.) ingress.

 

[hightower]

We don't need a DP isolator/switch - but why might we fit one?

The only reasons I can think of is for a means of isolation (eg for maintence). I'm sure I'm missing something with you mentioning TN-C-S again but I've drawn it out and can't see what.

Only other thing I can think of is an outside light might be subject to abuse by water - I've come across a few that are old and full of water. One had a single pole switch but kept throwing the RCD - the water must have been creating a neutral to earth fault and the SP switch couldn't isolate. At least with a DP switch you can isolate the light should it fail, thus keeping the rest of the house powered until it is fixed.

What am I missing?

 

[hightower]

@Karin has beat me to it again, but it seems we're thinking along the same lines - is that all there is to it @happysteve?

 

[happysteve]

Hi guys,

You've both got it spot on. A DP switch will isolate the exterior light in the event of a N-E fault, which would otherwise take out your lights and half the circuits on your house. As you've both clocked, faults are more likely due to environmental conditions. Of course, when installing the light you'll take account of these by using proper materials, but in fairness many exterior fittings are pretty crap. Imagine it's the middle of winter and it's blowing a gale, the last thing you (or the householder) wants to be doing is climbing up a ladder in the dark and the rain to try to isolate something. For the sake of a DP switch (or switched fused control unit, as they are pretty much always double pole switches nowadays) you can safely and conveniently get the power restored to half the house, and fix the light another day.

I suppose the other reason might be if the client wants to utilise the "permanent on" setting that many PIR-controlled floods employ. You turn it on, then off, then on again, and it will be permanently lit. Giving them a switch (of any sort - DP doesn't matter) will allow them to do this.

Good stuff!

Edit: Yeah, TN-C-S isn't really relevant here... but it's good practice to always be aware of what earthing system you're dealing with.

 

[Paul.M]

Good thread Steve, like it.

 

[happysteve]

Good thread Steve, like it.

Good to see you back, Paul.

 

[Paul.M]

Good to see you back, Paul.

I'm floating around just like a brown log in the bog.

Taking some time off atm, more important things in life has taken over.