Outbuilding consumer unit

"Greater safety afforded due to additional protection, twice. "


I`d question twice Zoom lad.


Theoretically say if you take a 7% failure rate then two cascading RCD faults might mean 0.49%.

That would be the expected maximum protection though.

If they are in the same enviroment and of same make and class they might both fail for similar reasons so I think difficult to achieve. but perhaps better than twice the chance of one RCD

No, that thought is again ridiculous. You try it, you cannot do much with a garden tool to a catenary unless it is a powered hedge trimmer! This "thing" about T&E and sunlight is a red herring too, I have seen cables at least 50 years old in this position with only some minor surface roughness from UV damage. NOT a problem. Hituf is probably better from a UV perspective as it is Black, but it depends on the exact formulation of the sheath, black actually absorbs more UV than grey or white cable. You would be happy with SWA in the sun, why not T&E?


Now the outbuilding. Can the lighting circuit be overloaded, reasonably not? It is possible to overload (slightly excessive current presumably 32A in a 27A rating) but is it dangerous? Ok we calculate the maximum cable temperature rise as about 50 degrees, and choose the likely cable temperature, in winter probably about 10 degrees. So now explain the danger? In summer why would you use 2 off 3kW appliances for a significant period? I would say not and give it a C3, it requires improvement, and suggest fitting a 2 way CU, Whether an RCD is fitted too is a matter of choice, but there will be no discrimination with the indoor one. As Z says, possibly slightly safer in case of a failure.


Again there is a distinct overestimation of problems and choice of very unlikely worst cases.

If you felt the urge you could swap the garage light switch for a switched fused spur, and satisfy the gods and eliminate a lot of un-necessary complexity and work, and make the sockets a lollipop ring ... I presume it does not need emergency lighting and has no bondable services like gas or water pipes.

nothing about it sounds particularly bad as it is, if they are striking the cable a lot, then move it up, but not to bother otherwise.

Mike.

With every respect for your great knowledge, Dave, may I disagree with you and defend the code?

The purpose of a fuse, breaker, or whatever is not just is not just overload protection but also short circuit protection.   Whilst I agree that the lighting circuit is unlikely to be overloaded (though people have been known to plug irons into light sockets); there is a very real danger of a short circuit when a man with a hammer and a bag of nails is at work hanging pictures.


And it is my rule 2 for d-i-y installation work that all wiring should be protected by a breaker smaller than the current rating of the wire..

Because I wa trained in an industry where fault discrimination on a large scale was very important I have a revulsion against putting two equally rated protective devices in series.  And that does extend to plug top fuses though I have never a correctly rated plug top fuse blow in this century   I will fit a two way CU for the building


A sharp spade carried at the half port could damage a T&E cable.which would be the time to make an improvement

"The purpose of a fuse, breaker, or whatever is not just is not just overload protection but also short circuit protection"


I`d modify that a bit.

An OPD (OCPD) might be used for one or more of three reasons:-

1/ Overload.

2/ Short circuit

3/ Earth fault.


Other methods might also be employed instead of or in addition to such. RCDs , limited loads by design, are two examples

This "thing" about T&E and sunlight is a red herring too, I have seen cables at least 50 years old in this position with only some minor surface roughness from UV damage. NOT a problem.

T&E can indeed be fine, but also it can degrade quite rapidly in sunlight. It all depends on the exact PVC formulation the manufacturer used at the time. In olden days (when I was young) there was indeed no problem and running T&E outdoors, clipped to walls and hung from catenary wires was perfectly normal - and indeed guidance said so. Then, if the story I've been told has any truth to it, someone noticed that some T&E was degrading quite badly and took it up with the cable manufacturers - who it seemed had changed the formulation - but they refuted the claim, pointing out (entirely accurately) that the BS for T&E cables had no requirement whatsoever for UV resistance. Realizing the implications of the problem IEE then does an about-turn and revises the guidance not to assume that T&E has the necessary UV resistance for use outdoors.


So for new installs we shouldn't use T&E in situations exposed to sunlight unless we can find some manufacturer's data stating it is suitable (which mostly isn't there).


For EICRs it's a case of it might be perfectly OK or it might be awful - with no way of telling other than examining the individual cable for deterioration (which in theory should be happening anyway of course).


   - Andy.

The question of short circuit protection needs a reference to the adiabatic equation, where you will find the lighting circuit (even if 1mm2 cable) is safe from that point of view on a 32A Circuit breaker (presumably type B although you have not mentioned it). So it is very unlikely that even a short would cause the cable to fail, and even if it did due to some other defect the result is not catastrophic. To have a "Dangerous" or Potentially Dangerous" situation is not quite so easy as is often imagined by the inadequately trained or experienced. It is exactly this that I keep discussing here. Your installation can be improved and is not to the 18th edition regulations, but even so could be quite reasonably described as requires improvement, a C3. A C2 or C1 needs more than non-compliance, it needs a fair level of risk that someone could be killed or injured by the defect. Such a defect, which has not been found in your case is an open circuit CPC because the primary level of protection is not reached, but as you have a 30mA RCD might not be considered immediately "Dangerous". The primary safety measure is "Automatic disconnection of supply" which should be achieved by your RCD, in exactly the same way as with a TT installation.


The idea above to change the light switch for a switched fused sput unit is reasonable and could be the first stage of improving your installation. The second would either to change the socket wiring to 4mm, to complete a ring circuit, or install a small CU with a 20 or 25A MCB for the socket circuits. Whilst doing this addition a 6A MCB for the lights might be the easiest solution for the lighting circuit. You may choose to add an RCD too, although the discrimination with the house might end up being a problem as I mentioned above. This could be sorted by adding an extra small CU, perhaps with a time-delayed RCD, or a non-RCD circuit to the existing CU, as RCD protection of the catenary is not a requirement unless the cable to it is buried less than 50mm in a wall, and is therefore not visible and reasonably protected from wall fixings.


You can see from this list that the corrections are not very onerous or difficult, and potentially need no work inside the house, depending on the various options you take. The outside works in the outbuilding would take a good electrician an hour or so, and 15 minutes to test fully a bit more if you change the cable size or make a ring, assuming the wiring is in mini trunking or clipped direct.


A good way to check if EICRs are reasonable is to pretend that the Inspector is charged with fraud because his EICR is too extreme and he wants the work to correct the installation. As this is criminal, the level of proof of his findings will be "beyond reasonable doubt", not the civil "the balance of probabilities". Each of his findings would be cross-examined using an Expert Witness for soundness, to show that they were fraudulent (or not). Remember that the Inspector has to show that they are fair, "beyond reasonable doubt". This reversal of the usual position that most Inspectors may fear is that they were not severe enough, because they doubt their own judgment and their ability to justify it. Surely this is a good test of competence?