Spur to a shed

[Andypease1]

Is it accetable to run from a spur to a shed using 2.5 T and E cable in steel conduit securely clipped to concrete posts and concrete boards. Assume an earth clamp to the conduit to a sutiable earth point is required.

Cheers

 

[EddieM]

No, use SWA, how far away is the shed.

 

[sparkwright]

You again.

The usual way to connect metal conduit to earth is not by an earth clamp.

At the house end it may be best to use a metal conduit box, with an earth lead from the box connected to whatever your supply is. Typically a round metal conduit END box with a 20 mm hole at the back is used outdoors. The back is fully sealed with waterproof compound or similar, and a gasket used behind the lid.

At the garage end it's usual to do it in the same manner as above, or to run the conduit to a metalclad consumer unit, or metalclad socket as the case may be. You need to be certain that the conduit and metalclad enclosure are fully earthed.

That said, you really should be looking at SWA cable.

 

[Andypease1]

about 40 feet - i only ask because that is what is there already. Nice tidy job shame to rip it out. realy tough conduit has been used with threaded joints.

 

[Taylortwocities]

Plus…
How is it connected at the house end, you say its a "spur".
Does the cable come from a fused connection unit, or is it just connected into the back of a ring final socket?

Is this related to your , garage electrics query in any way?

 

[AndyPRK]

about 40 feet - i only ask because that is what is there already. Nice tidy job shame to rip it out. realy tough conduit has been used with threaded joints.

Certainly how it was done in the old days. Could well of been complient at the time of install.

As you say its a shame to rip it out if it's fit for purpose. Just need to be sure there is good earth continuity at the shed or fit new earth conductor.

It's well worth bringing the circuit protection upto date though if it's connected to a ring. And ensuring sockets are rcd protected.

 

[Andypease1]

Thanks guys

From the back of a socket but could change that

Good earth in shed but need to earth the conduit.

Nice little CU in shed with circuit for the light and one for the two sockets

 

[ericmark]

At the house end if house already RCD protected I would add a fused connection unit (FCU) if not RCD protected a RCD FCU I would also test the loop impedance in the shed if the loop impedance is within limits then I would leave as it is.

Impedance to comply with volt drop should be better than 1.5Ω and to comply with 13A fuse 2.42Ω but if RCD protected although really should be within volt drop the earth loop impedance can reach 200Ω before considered dangerous.

 

[ban-all-sheds]

Good earth in shed but need to earth the conduit.

If it isn't earthed it was not a "nice tidy job".

Nice little CU in shed with circuit for the light and one for the two sockets

Pointless.

 

[JohnW2]

Impedance to comply with volt drop should be better than 1.5Ω and to comply with 13A fuse 2.42Ω but if RCD protected although really should be within volt drop the earth loop impedance can reach 200Ω before considered dangerous.

AFAIAA ... as far as the regs are concerned, other than in TT installations, RCDs are only meant to provide 'additional protection', and therefore shouldn't really be relied upon to satisfy fault protection requirements in situations in which one can't be bothered to install sufficiently large cables.

Kind Regards, John

 

[Adam_151]

411.4 TN system

...
...
...

411.4.4 The following types of protective device may be used for fault protection:
(i) An overcurrent protective device
(ii) An RCD.
NOTE 1; Where an RCD is used for earth fault protection the circuit should also incorporate an overcurrent protective device in
accordance with Chapter 43.

Its not considered good design practice, however it is allowed

 

[JohnW2]

411.4 TN system ...
411.4.4 The following types of protective device may be used for fault protection:
(i) An overcurrent protective device
(ii) An RCD.
NOTE 1; Where an RCD is used for earth fault protection the circuit should also incorporate an overcurrent protective device in
accordance with Chapter 43.

Its not considered good design practice, however it is allowed

Yes, we've often discussed that, but it all gets rather confusing when one tries to square it with:

415 ADDITIONAL PROTECTION
415.1.2 The use of RCDs is not recognized as a sole means of protection and does not obviate the need to apply one of the protective measures specified in Sections 411 to 414

...since one then gets into some confusing circular arguments regarding the 411.4.4 you have quoted.

My personal feeling/interpretation of all this (based more on what I regard as 'common sense' than anything else!) has always been that, if there is some 'genuinely good reason' why adequate fault protection cannot be achieved (in a TN system) with an OPD, then it is acceptable to rely upon an RCD for this (although it then becomes 'primary', rather than 'additional' protection) - but that, as I said, I personally don't feel that not being bothered to install a fat enough cable really counts as such a 'genuinely good reason'. However, I'm sure that others will have different views about this!

Kind Regards, John

 

[Adam_151]

I'm sure it used to be worded better in the 16th....

I'm sure 415.1.2 is designed to disallow RCD protection as a subsitute for installing a CPC, as they sometimes do in forgein parts, granted it's not all that clear!

Going back 5/6 years when the commercial contracting world was going RCBO mad, quite often you'd only be able to get hold of RCBOs in C type, and and any protest that the design with out of spec zs was met with the response that we were permitted upto 1666 ohms on a 30mA. Gradually things improved though and type Bs became easily obtainable (except merlin for some reason).

 

[JohnW2]

I'm sure it used to be worded better in the 16th....

Maybe, I can't remember.

I'm sure 415.1.2 is designed to disallow RCD protection as a subsitute for installing a CPC, as they sometimes do in forgein parts,

That would seem fairly unlikely, given that there is already a reg which requires every 'point' in a circuit to be served by a CPC.

... granted it's not all that clear!

I think that's a serious understatement! 415.1.2 says that an RCD cannot be the sole provider of fault protection and that the presence of an RCD does not remove the need for one of the protective measures specified in sections 411 to 413 - but, as you have pointed out (411.4.4), one of the protective measures specified in those Sections is an RCD !!

The nearest to a sensible view of this I can think of is that they meant to write "...one of the other protective measures specified in Sections 411 to 413.." ... but who knows!

I still rather doubt that the intention was to allow an RCD to be the sole device providing fault protection unless there was a pretty good reason why it could not be achieved with an OPD.

Kind Regards, John