Garden shed - power from the house

[7170]

Hi, I've recently moved house and have some questions on the existing wiring setup for the cable to the shed.

The house has had a recent extension but the certificate and documents do not explicitly mention the cable to/and the shed so I can only assume that was not tested at the time.

The shed is wooden, about 30m from the house, about 1.5m lower, on a concrete slab with wood bearers though like any wooden shed the outside of it will be damp after rain etc. The ground here is mainly clay below about 45cm.

The cable is 4mm 3 core armored cable. At the consumer unit in the house it is connected to a 16A MCB which is protected by a 30mA RCD. Both the earth core and wire armor are connected to the earth rail in the box. The house has TN-C-S earthing.

Now, the shed setup is different to what I've had in past places. The armored cable comes into the consumer box in the shed but the earth and outer wire of the armored cable are not connected to anything. There is an earth rod instead and this is the sole source of the earth in the shed.

I have three main questions, the first, is this a setup that is safe and acceptable with wiring codes (it was done several years ago I believe).

Secondly, the consumer box in the shed also has a 30mA RCD which covers the two circuits in there. I was under the impression you should not put RCDs in series, is the fact the earth in the house and shed are a different type the reason for this, or is it just incorrect?

Finally, the earth rod is outside the shed, inline but obviously below the consumer box, it is not next to the path but I think there is the risk it could be damaged by footfall / the mower etc, so I think it may be prudent to move it along slightly to a safer location, are there rules about how far the earth rod should be from the box or not?

I guess I am trying to understand if it is unsafe as it stands and needs an electrician to do things correctly, or is it in fact acceptable and safe? Many thanks in advance for any assistance.

 

[westward10]

You sound like you have some electrical knowledge but say you have no electrical qualifications.

 

[7170]

You sound like you have some electrical knowledge but say you have no electrical qualifications.

I'm into radio astronomy (at an amateur level) and over time with that comes some know-how of electronics but certainly no formal electrical qualifications or knowledge of electrical regulations and correct mains earthing approaches alas.

 

[The Ghost]

In principle it is safe. I am of the persuasion that exporting TN-C-S is questionable so TT is the way to go if you look at the resource below it will shed light on this.You can have RCDs' in line but the upstream RCD should be S type (providing discrimination) to make sure the RCD in your shed trips first. Of course there is the question of all wood structure, it complicates things and I would say that is a flaw in the setup. The upstream RCD protects the supply cable from being damaged. The earth is separated to make sure the cable is protected where it is buried. Hence I would not connect the earthing of the outer armour or earth in the SWA to the discrete system in the structure/shed. The earth rod should be contained within a box set into the ground to avoid the risk of what you are anticipating. There is a resource on this forum about exporting TN-C-S which will give a lot of insight into the working of such processes. If you click on Forums above it can be seen to the right a little way down with stars next to it.

 

[Wilko]

Hello and Welcome Mr 7170
Once you've had a read the info V has suggested, perhaps have another butchers at the installation and see if it fits. Do check RCDs firing with the button test.
Edit : the exposed (unlabelled? ) earth rod situation may need a fix.
Do the docs you have include a schedule of tests and results? Perhaps share these and any pics and we can comment further.

 

[Des 56]

The set up you have errs on the side of caution
The Tnc-s earthing system of the main installation could in certain situations become a weak link( regards safety) Tnc-s earthing systems have been known to be extended outside the equipotential zone of the property to perhaps feed a shed/garage etc,that shed may have extraneous conductive parts( via its structure, or maybe services in the shed) with lack of the required main bonding back to the house,this whilst also having true earth near by or in the vicinity,not a good mix

The armoured cable feeding your shed need have no need of Rcd protection, if it has,it should be a timed delay Rcd in order to discriminate between the shed and the house,after all you don't wont the house tripped because of a problem in that shed
The damp shed in itself may only be a problem if it gets into accessories and has no bearing on the supply or the protection required
Other than a possible nuisance of a less convenient Rcd tripping during a fault.you don;t seem to have any issues that are of concern

 

[7170]

Vortigern, Wilko, and Des 56, many thanks for your replies, very informative and highly appreciated.

Wilko, the test certificate I have clearly indicates which circuits were tested when the extension was put in and the shed was not one of them, hence my assumption it was not checked at the time.

I've had a look again and the RCD in the house consumer unit is not a time delay unit and the earth rod will need to be looked at as it is not labeled or in a covered pit, plus I still think the location is poor.

We will be having some work done in spring to i'll ask for these two things to be looked into and remedied at the same time. The house consumer unit is full so there is no more room in there for a time delay RCD too, but I think a bigger one with more ways will be needed anyway.

Good to know that it isn't in a critical state at present requiring immediate action, and I've tested the RCDs and they are all working when I press test too.

Many thanks again.