Finishing the Garage off with lights + RCD question

[Chamonix]

Chaps,

Having read some of the other posts on here, I thought I would ask a question.

Last year, bought a total wreck of a house, and re-wired the entire lot. First time for me - but I looked it all up carefully, complied with regs (I hope!!) and wired in:

1. New Consumer Unit - with 100A, 30mA 2-pole RCD main switch and 8 MCBs
2. RCD - separate unit, between the mains and the consumer unit itself (same capacity - installed by electrician - allowing me to install new consumer unit et al)
3. New power rings (upstairs, downstairs, kitchen)
4. Lighting radials (up and down)

Having now built a garage (phew! hard graft) I have added another ring main for the garage - again as a ring, using 2.5mm armoured cable (which is terminated into connectors to join to regular 2.5m cable as the armoured stuff is sooooo stiff). As this didn't exceed my maximum (68m?) cable lengths and I had to dig a trench anyway (4 metres long), thought easier to just do a ring.

Now - question is - currently I just have some add hoc lights in the garage, plugged in halogen lamps. My thought would be that the easiest way to wire the lights properly would be to take a spur off the ring, to a fused connection unit, and then to the lights (to be purchased). However, I have never seen anything in writing to say that it's OK to take lights off a ring main.....

This seems easier than anything else. Questions are:
1. If I want say, three lights in the garage, same switch, should I just use the fused spur connection unit as the switch?
2. How should I wire the lights together - as a simple radial back to the connection unit?
3. Do I need to consider anything special when buying the lights?
4. What size fuse in the connection unit?

I imagine that I would use 2.5m cable to the connection unit and then 1.5 for the lights thereafter?

Last question. If I effectively have two RCD's protecting the entire house (one in the separate unit and one being the double pole switch in the consumer unit) - would it matter that the one in the consumer unit doesn't seem to trip when I hit the "test" button which I checked yesterday? The other RCD works fine on "test", but the one in the consumer unit appears not to, and it's only a year old......

Sorry to ask so much, but better safe than sorry... I can always replace the double pole switch whilst I am at the rest of the task and the power is off......

Ian.

 

[plugwash]

putting everythinhg on a 30ma rcd is considered bad practice because you loose your lighting if it trips i don't think it breaks any regs thoguh

running a ring to a garage like that is unusual to say the least ...
there should really be local isolation in a garage bearing in mind you have already wired it as a ring the easiest way to do this would be to use a 4 pole isolator to isolate both legs of the ring at once something like http://www.tlc-direct.co.uk/Main_In...gear_Index/Wylex_TP_and_N_Isolator/index.html
however if you do this you must make sure there is nothing in the ring legs before they reach the isolator

runnign lighs froma ring with fcus is not a problem but you must use 2.5mm for all wiring before the fcu

 

[Big_Spark]

If any RCD/RCBO does not operate when the TEST button is depressed, then the unit is knackered and should be replaced as a matter of urgency.

I would suggest an alternative though, as there is an RCD between your board and the mains, there is no need to have an RCD as your main switch, sonsequently I would buy a 100A Switch Disconnector for your Board and ditch that knackered RCD. Then with the saved money, I would buy a single module 32A 30mA RCBO for the garage ring.

 

[Chamonix]

Plugwash,

Thanks. But if this is no different to any other household ring, why would you need to? It's easily done, there is somewhere for it to go directly once it enters the garage before either leg, I thought it might be needed so I left space - so easily added.

Is the local isolation just to make it easiler to turn it off, or is it a requirement?

BTW - why so odd to have a ring rather than a radial? aren't they less fuss and you have to have the trench anyway, so why not lob in two cables instead of one?

Thanks for the info on the lights. Any idea what size fuse? 3A (for three halogen lights?)

Ian

 

[Chamonix]

FWL - thanks for that. I thought I would have to replace this now, but have to say - not impressed that it bust in under a year (it's a brand new Wylex consumer unit)....

Plugwash also made the point that it wasn't great practice to have a 30mA as the switch as you lose the lights....noted - that has happened every time a bulb goes, but that's how it came....

As the entire thing is behind an RCD, why use a separate RCBO for garage ring? This is the same question I just asked Plugwash on the separate local isolation in the garage?

I am a little confused now.....but still ****ed at Wylex for having to replace the RCD switch after 11 months.....

Thanks,

ian

 

[plugwash]

nah not really a lot of point in haveing two rcbos of the same type in a row

as for local isolation in the garage every building is supposed to have its own means of supply isolation

if you are going to isolate a ring then you need to isolate both legs with the same switch so it cant be left with one connected and the other not hence why i suggested that 4 pole isolator

 

[Chamonix]

Clearly I have both cables entering the garage, then they separate at the junction box and go around the room.

Is the four-pole wired up as I would guess - i.e. that you simply connect all four live to the switch and all four neutrals to some kind of bar inside that box, presumably that box looks like a little consumer unit then for all earths to connect up to aswell?

Good value though at under £20. Will do at the w/e if it's simple to wire up (don't have forever to mess around this weekend....)

Ian

 

[Big_Spark]

nah not really a lot of point in haveing two rcbos of the same type in a row

That is purely dependent on the rating of the primary RCD in line. I would suggest that this is checked out as it is quite likely to be either a 100 or 300mA unit. That being the case, installing a 30mA unit for the Garage services not only allows discrimination to occur, but complies with BS7671.

as for local isolation in the garage every building is supposed to have its own means of supply isolation

if you are going to isolate a ring then you need to isolate both legs with the same switch so it cant be left with one connected and the other not hence why i suggested that 4 pole isolator

Plug, this is a domestic installation, and there is nothing wrong with the circuit as proposed. The circuit has the ability to be isolated. OK, if you want to be pedantic then installing a method of isolation in the circuit inside the garage would do not harm, but is not really required in this set-up.

 

[Chamonix]

For the lights?

3A, then connect all three in series as per standard radial lighting?

Pardon my terminology if I am off - but I hope you know what I mean.

Thanks,

Ian

 

[Big_Spark]

Cham..yep, simply wire from a fused spur to the lights in series. The Switches are connected in series with the feed to the individual light..If Ya know what I mean

 

[Chamonix]

its appreciated.

will order the last bits now, and add on a new switch for main consumer unit. £7 rather than £49 for the one that broke.....let's me get the RCBO for just garage ring. Much cheaper.

Thanks.

Ian

 

[ban-all-sheds]

Have I missed something, or has nobody considered the possibility that this is a TT installation, and that the first RCD is providing the whole-installation 100mA protection, and the one in the CU providing the 30mA for the sockets (and everything else as it happens, but that's by the by).

If so, the CU RCD should not be replaced by a simple incomer, unless every socket MCB is replaced by an RCBO.

If it's not TT, then why on earth were there ever 2 RCDs in series, and given that there is the first one, what would be the point of using an RCBO for the garage?

 

[kendor]

If any RCD/RCBO does not operate when the TEST button is depressed, then the unit is knackered and should be replaced as a matter of urgency.

I would suggest an alternative though, as there is an RCD between your board and the mains, there is no need to have an RCD as your main switch, sonsequently I would buy a 100A Switch Disconnector for your Board and ditch that knackered RCD. Then with the saved money, I would buy a single module 32A 30mA RCBO for the garage ring.

Not necessarily so, the test button causes an imbalance through a resistance to earth this is to make sure that a decent earth is present if the test button doesn't trip the RCD that doesn't mean that it is necessarily knackered.

 

[plugwash]

most non dc sense rcds dont have an earth conenction
i thought the test button connected between one terminal of the output and the other terminal of the input (via a resistor ofc)

 

[Big_Spark]

Not necessarily so, the test button causes an imbalance through a resistance to earth this is to make sure that a decent earth is present if the test button doesn't trip the RCD that doesn't mean that it is necessarily knackered.

Kendor, they actually cause an imbalance between the phase and neutral. Many years ago they used an imbalance to Earth, but they stopped them about a decade ago or so.

The DC sensing ones actually monitor the earth for any DC currents that may be present, and if they occur above a preset level, the electronics then cause a phase / neutral imbalance that results in operation of the RCD.

I confirmed this last week with the Tech Guys at MEM when I was doing the web pages for AE3 as I was not 100% sure which method they used myself. A follow up call to MK also gleaned the same information.