Exporting TT earth to detached garage?

[Flinstone]

The supply to my house uses a TT earthing arrangement. The incoming supply to the house CU (fuse wire holders) is via a standalone Wylex WSES 63/2 63A 30mA RCCB.

Seperate but adjacent to the house CU is a 30A FCU and another Wylex 63A 30mA RCCB that supplies the detached garage CU 11 metres away. Wiring from the RCCB to the garage is via T+E indoors and then MICC/Pyro cable outside.

The outdoor cable is routed to a shed via a double-socket/lighting FCU and then onwards to the garage. The garage CU is therefore not fitted with a RCD, instead it relies on protection from the house board. The shed has no RCD either of course.

I now intend replacing the MICC with 3-core 6mm SWA. Since the earth is already exported, should I continue to keep it this way or isolate the earth and install a local earth rod for the garage? What about the shed too? Perhaps someone could explain the merits of either method to me please?

On a seperate note, should I remove the 30mA RCCB in the house dedicated to the garage and shed, and instead install two RCDs locally within each outbuilding? I can appreciate the merit of it discriminating (is that the correct word?) between shed & garage but I would like to keep cost and hassle to a minimum, assuming it is safe to do so, especially since at present I am only replacing the cable and not changing anything at the CUs (which would be notifiable).

Thanks folks,

 

[Chri5]

I'd consider the work notifiable anyway !

Best method is exporting the earth via a core of the swa and the armour.

Best to feed swa via a henley and direct from the main supply rather than via a fuse in the house CU.

In honesty, I'd jack the job as a DIY and get a sparks in. Provided the route for the cable is clear, safe and done to regs you should find a spark who will do the tech and test side. You doing the physical run, clip etc.

 

[ColJack]

what's the point in exporting a TT earth? surely sinking another earth rod, much closer to the shed, is better than going back to an earth rod somewhere else?

 

[Taylortwocities]

what's the point in exporting a TT earth? surely sinking another earth rod, much closer to the shed, is better than going back to an earth rod somewhere else?

Here here. I would never, ever export a TT earth.

This famous article details the appraoch, and details a TT installation.
Note that this was written pre-17th edition but it is still largely relavent.

 

[mfarrow]

This famous article

 

[Taylortwocities]

Oh poo.

Here it is, http://www.theiet.org/publishing/wiring-regulations/mag/2005/16-elect-inst-outdoors.cfm?type=pdf

 

[nozspark]

and also all circuits in a TT installation require RCD protection... no matter what!

So don't just remove the RCD

 

[ban-all-sheds]

If you're going to have a local rod then do not also connect the exported house earth at the outbuilding end, and terminate the SWA in an insulated enclosure.

 

[Flinstone]

Thanks for your input & the helpful pointers!

I'd consider the work notifiable anyway !

On what basis is replacing the cable considered notifiable?

Best to feed swa via a henley and direct from the main supply rather than via a fuse in the house CU

Why is (or perhaps could) the fuse be a problem?

what's the point in exporting a TT earth?

Because the installation already exists as such. Doesn't make it right of course. Have 6mm 3-core XLPE SWA waiting in the wings as a replacement. Obviously I'd prefer to keep cost & labour to a minimum and replace only cable, where this is acceptable, since on a limited income. Added cost of an earth rod, g/y wire, replacement garage CU, etc doesn't excite me like perhaps it does the pros

So is CONTINUING to export the TT earth to a detached garage unacceptable?

From what I know from the previous owner, the garage supply was installed in 1986. Was exporting the earth from a house with a TT system acceptable back then? And have the requirements/recommendations since been modified? Or was the electrician wrong in the first place? Would just like to understand a little more about this.

Oh poo. icon_redface.gif

Here it is, http://www.theiet.org/publishing/wi...5/16-elect-inst-outdoors.cfm?type=pdf[/QUOTE]
Thanks for the link.

and also all circuits in a TT installation require RCD protection... no matter what!

So don't just remove the RCD

Agreed! But for my particular setup described (RCD at source in house), you say don't remove it. Can you confirm there should be NO RCD protection within the outbuildings (garage/shed)? Thanks.

If you're going to have a local rod then do not also connect the exported house earth at the outbuilding end, and terminate the SWA in an insulated enclosure.

Understood and thanks for stating.


Kind regards

 

[Electrifying]

On what basis is replacing the cable considered notifiable?

On this basis:

I now intend replacing the MICC with 3-core 6mm SWA.

It's a totally different type of cable with totally different characteristics.

This is basically a distribution cable to two outbuildings.
It's hardly minor-works, is it - you're gonna have to do a full set of tests in the shed and garage when you're finished.

Page 8 - Approved Document P:-

Additional Notes

f. Detached garages and sheds are not special locations. Work within them is notifiable only if it involves new outdoor wiring.

You cannot use the fact that you are replacing an existing cable as there are conditions - i.e. -

Approved Document P pg8, Table 1

Not notifiable
Replacing the cable for a single circuit only, where damaged, for example by fire, rodent or impact.
The replacement cable must have the same current carrying capacity and follow the same route

As you are also talking about altering the positions of protective devices, I would have to agree with 'Chri5' and say that the work you intend to do is notifiable

 

[Paul_C]

So is CONTINUING to export the TT earth to a detached garage unacceptable?

It's perfectly acceptable, and always has been.

There seem to have been a growing number of myths about how "exporting" earths from one building to another is somehow unacceptable, which has led to the increasing use of separate TT in outbuildings, which in turn seems to be perpetuating the myths.

There are good reasons for keeping everything securely bonded together and not providing a separate earthing arrangement for outbuildings, even though the IEE Regs./BS7671 permit it (and in fact in the electrical codes in some other countries, a separate earthing system not bonded back to the main house earth is not permitted).

 

[Flinstone]

Many thanks indeed Paul_C !!

As a matter of interest can you advise if it remains appropriate to have the RCD (RCCB) at the house end and therefore no RCD at garage or shed? (My installation is TT earth exported out to to shed and garage, RCD at house end only). In fact is this the only suitable method for a house using a TT supply, as I'm not clear if the distribution cable should be protected too for this system, bearing in mind my distribution runs in T&E around the roof space of our kitchen extension before changing to armoured (so can be accessible - no mechanical protection on the PVC portion).



Thanks again

 

[LoveRocket]

EVERYTHING in a TT installation must be RCD protected, but not necessarily at 30mA. That's why most TT installs have a 100mA type S (time delayed) RCD incomer.

So yes, the run to your shed MUST be RCD protected, so having the RCCB before the switchfuse is acceptable, although it means that should it trip you'll lose the lights in the shed and have to go back to the house to reset it.

Personally, I'd go

100mA type S RCCB - Switchfuse - cable run - garage CU with an MCB for lights and a 30mA RCBO for sockets.

Then you'd have the cable run with 100mA RCD protection as required for a TT, and a fault would only take out the sockets in the garage.

 

[Paul_C]

As a matter of interest can you advise if it remains appropriate to have the RCD (RCCB) at the house end and therefore no RCD at garage or shed?

With TT, you'll always (practically speaking) need RCD protection at the supply end of the distribution cable, because the earth resistance is relatively high and the RCD is necessary to provide earth-fault protection for the cable.

If you're stuck with TT, then the best solution is to use a type S (time-delayed) RCD of 100 or 300mA rating at the house, then install 30mA RCD's at the outbuildings as needed. If you have a regular 30mA RCD at the house end then everything will be perfectly safe, just more inconvenient when a fault somewhere outside trips out the power to both shed and garage and you have to trek back to the house to reset the RCD.

Putting an RCD of a similar type at the garage/shed end would serve no useful purpose as the whole feed would already have 30mA RCD protection from the house, and there's no guarantee that the garage/shed breaker would trip before the house one.

Although a single, sensitive 30mA RCD at the house end provides perfectly good safety, you could start getting problems with nuisance tripping with too much connected on it, especially with longer cable runs to outbuildings.

Edit: Cross-posted with above.