RCD or no RCD

DinoTheDino · 12 Feb 2016

Hi all,
i want to wire up my shed (a double socket, one socket for the freezer, the other for the occasional lawn mower, pressure washer etc. and a light). The house is RCD protected at the CU.
I have been told, i can start from the living room socket (feed), to wire up a RCD spur to the socket and from there with a minimum of 2.5mm twin and earth (which is going to be an outdoor cable, in conduit) to the shed.
I would additionally (without being told) install a neon control switch between the feeding socket and the RCD spur to be able to isolate the shed without needing to switch off all the downstairs sockets.
My question: Do i need the rcd spur considering the living room socket itself is rcd protected? I am afraid the rcd "might" trip at some point and the freezer in the shed will be off without me realising it; as well as, what is the situation when a rcd spur is wired up in a rcd protected circle (or ring?)
Would a MCB on the CU be sufficient and safer considering the rcd, to my knowledge, provides no protection to overheating? (this option would mean a lot more work, but if it comes down to safety i don't mind)
As you can possibly tell, i am not an electrician, but have a good understanding of technical matters and a "certain amount" of common sense

. I am also aware, Electrics is not a playground!!!!!
The above terms (MCB,CU) are discovered through my own research in the internet.
Any reply would be greatly appreciated and pretty please, try not to use electricians jargon

Thank you all in advance and kind regards
Dino

Taylortwocities · 12 Feb 2016

If the circuit is already RCDprotected then there is no point having a second RCD (RCD spur).
In the evnt of a fault in the shed, the RCD spur may trip, or the house RCD, or both. It depends on the size of the fault and the point on the AC sine wave that the fault happens.
If you absolutely do not want the freezer to defrost if there is a fault in the shed then a different approach should be taken.
But, quite honestly, your house circuit is just as likely to trip because af a juicy sultana dropping into your toaster from your hot cross bun!

An isolation switch for the shed circuit is necessary, use a regular switched FCU with a neon (if you want one). That's all you need.

Re

DinoTheDino said:
Would a MCB on the CU be sufficient and safer considering the rcd, to my knowledge, provides no protection to overheating?

That is the better way of providing the circuit, but it is a much bigger job!

The shed would need an RCD of its own, in that case. Do note that RCDs and MCBs do different jobs. MCBs protect against over current (NOT overheating!). An RCD protcets against earth leakage problems such as may be encountered when you run your lawn mower over its electric cable.

More…..
I guess this is a new circuit to the shed? How will you run the cable, What sort of cable did you have in mind?

DinoTheDino · 12 Feb 2016

Hey, thank you for the swift reply.
As stated, 2.5mm twin&earth outdoor cable and i was going to use 20mm conduit as well, all the way to the shed, then a split box(?) to divide it to a light switch and the socket.
I think i meant over current(?) in regards to the MCB, meaning if i got the pressure washer running at the same time as the lawn mower, if that is going to be "too much" for the socket.
See below the pictures of my CU

When you say the shed will need its own RCD, do you mean feeding from the living room socket to the shed, without RCD, and before the shed "split" as i mentioned above , i install an RCD spur???
Thank you again

DinoTheDino · 12 Feb 2016

obviously with the FCU installed after the living room socket....

Taylortwocities · 12 Feb 2016

To run the circuit directly from the consumer unit would mean serious reconfiguration of the consumer unit.
Not a job for you.
If a new circuit is created this is notifiiable work, so you would need to call the LABC and pay their fee, before you start. Or get a registered electrician to do this for you.

Back to the real option.
How will this cable be run to the outside shed? Note that twin and earth cable is NOT suitable for use outside.
If it is to be buried, you need to use steel wired armoured cable, buried at a suitable depth, so i hope you like digging.

The maximum current that you can draw will be limited by the fuse in the FCU in the house, so 13amp. May be enough for pressure washer+lawn mower, but depends on the rating of these devices.
So the cable could go to a double socket, that will be fine.

Then the lights…. you will need to fuse the circuit down for the lights. best option is to have a switched FCU by the door with a 3A fuse in it.

DinoTheDino · 12 Feb 2016

Hmmmm,
yes, i was aware of the "armoured or not" options, and i found this cable on, -sorry- ebay, which states:

NYY-J PVC HI-TUF BLACK 2.5mm 3 CORE OUTDOOR CABLE
so i thought in a conduit it would be fine, and yes, partly buried, but in conduit all the way. And i must say, i think i gathered the difference between twin and earth and 3 core now!! The twin and earth is where the earth is a naked wire needing a yellow and green sleeve(?).
Anyway.., so for me to resume:

Off the living room socket, NO RCD spur, to a FCU, from there 2.5mm 3 core armored cable in conduit ( i prefer it in conduit regardless if needed or not), buried to the "required and safe" depth to the shed. Then split the cable to 1: 3A switch and then to the light AND 2:to a double socket. Finito!

Am i on the right path? And the RCD is not required?? or did you mean RCD still needed in the shed (after the split in the shed and before the double socket)

Sorry to bug you so much Taylortwocities, i am trying to save and at the same time to be "reasonably" safe

Muchas gracias

chivers67 · 12 Feb 2016

Just as side note, frost free freezers can tend to stop running if it's freezing outside they're used to being in warmer ambient temperatures so if you haven't already purchased the freezer make sure it isn't frost free. http://www.telegraph.co.uk/finance/...owell-do-freezers-need-to-be-kept-inside.html Have a read of this.

chivers67 · 12 Feb 2016

Also https://www.tlc-direct.co.uk/Produc...zUsFFwf3zi6_WHlbD5SaBYXiPgOFyEKONMBoC733w_wcB this needs to be installed above outdoor cables, how far is your run BTW?

DinoTheDino · 12 Feb 2016

Thank you both for taking the time.
Chivers67, the run would be 2680mm 90 degree bend, 500mm 90 degree bend, 1200mm to the shed, I assume BTW stands for "by the way".
The freezer exists and it's not frost free, thank you for the accurate and excellent tip!
Still not sure if I need the RCD or not in the shed, IF at all !
Regards

Taylortwocities · 12 Feb 2016

If the shed is energised from the RCD protected circuit

DinoTheDino said:
the living room socket (feed)

then another RCD is not required

DinoTheDino said:
the run would be 2680mm 90 degree bend, 500mm 90 degree bend, 1200mm to the shed,

Blimey, you aren't making a watch. Most of us just pace it out and say
"thats about 5 metres"

DinoTheDino · 12 Feb 2016

Well, sorry for being so meticulous

fair enough Taylortwocities, I thought every detail could be vital considering it's electrics and safety we are talking about. That's because I had the numbers still in my head from last week, I thought I show off a bit

Ok, I'm sure I got the answers needed and I thank you both very much!
Best
Dino

DinoTheDino · 12 Feb 2016

One last thing aboput the FCU, i hope....,

Taylortwocities said:
An isolation switch for the shed circuit is necessary, use a regular switched FCU with a neon (if you want one). That's all you need.

Taylortwocities said:
The maximum current that you can draw will be limited by the fuse in the FCU in the house, so 13amp. May be enough for pressure washer+lawn mower, but depends on the rating of these devices.

does it matter if it is 45A? As far as i understand it is "just" a switch!
The reason for me asking is, the 45A has a switch that requires a good amount of force to switch on/off whereas the 13A can be switched on/off by the press of a finger.
The location of the switch will be behind a cupboard, quite low, and i am afraid if someone is trying to access the (feeding) double socket next to it, might switch it off without realising it. Needless to say the adults in the house are going to be aware of it but you never know.....!
Thanks again and if i can help in a linguistic way, i would be glad to. Fluent in German and Greek, English is getting there

DinoTheDino · 12 Feb 2016

.....I mean Amp of course

PBC_1966 · 12 Feb 2016

DinoTheDino said:
does it matter if it is 45A? As far as i understand it is "just" a switch!

There is never a problem with using a switch which is rated higher than you need. The 45A rating is simply the maximum that the switch is designed to carry safely.

DinoTheDino said:
NYY-J PVC HI-TUF BLACK 2.5mm 3 CORE OUTDOOR CABLE
so i thought in a conduit it would be fine, and yes, partly buried, but in conduit all the way.

With the overall diameter of that cable, I wouldn't even try to pull it through 20mm conduit with multiple 90-degree bends.

DinoTheDino · 12 Feb 2016

Hi PBC_1966 and thank you for your reply!

PBC_1966 said:
There is never a problem with using a switch which is rated higher than you need. The 45A rating is simply the maximum that the switch is designed to carry safely

I am not an expert, but i'm not sure if US rules are the same as UK(?), unless i get an expert's view from the UK, my "question" is unanswered and this is not offensive meant!!!

PBC_1966 said:
With the overall diameter of that cable, I wouldn't even try to pull it through 20mm conduit with multiple 90-degree bends

...i understand what you mean but i wouldn't have thought that the cable is so thick, apart from that i would feed the individual pieces of conduit/bends one by one through and then connect them to each other..., does that make sense? In worst case scenario i will change to 25mm conduit which will make no big difference...!!

Thank you again PBC_1966

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RCD or no RCD

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