Garage Wiring 1970s - is this correct?

[securespark]

It may be a spur from the ring final.

When the conduit was installed, it was not a requirement to earth it. That is a recent change from the original Red book of 1991.

 

[ban-all-sheds]

Did the OP not say the conduit went underground ? Would that not be earthed then?

No, that would not count as being earthed.

 

[ban-all-sheds]

It may be a spur from the ring final.

I thought it was.

When the conduit was installed, it was not a requirement to earth it. That is a recent change from the original Red book of 1991.

Oh - OK.

When was an installation method for cables in underground conduit available, and when did the blanket de-rating for BS 3036 fuses come in?

 

[empip]

Wasn't there effectively a 0.75 derating at the 14th in '66 ( 'Coarse protection' tabulated ratings were allowed an increase by 1.33 when utilising 'close protection' )... Reciprocal of 1.33 ~ 0.75

From the knotted wiki

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[ericmark]

My oldest wiring regulations book does not go back to 1970’s but I did start in late 60’s and can remember some of the changes which came in just as I started and one was using conduit as the earth. Although we can use conduit as an earth now it was stopped for a time mainly as at that time earth loop impedance testers were rather rare so rust could and did cause bad earths.

Today as long as premises are tested at the prescribed intervals i.e. Domestic every 10 years and change of occupants then I see little problem. But so many householders do not get the premises checked.

So a 1970’s house should have been checked at least 3 times more likely 4 or 5 so there should be test results at least from date the present owners moved in. If not they should be asking solicitor why. My daughter did not get the electrics tested and this was because they were running short on time as original house had already been sold. However they were well aware that it had not been done and were also well aware of most of the faults.

Although the BS7671:2008 does give a limit of 3m see 433.2.2 and 473-02-02 in old 16th Edition how far back that goes I don’t know? But the cure is rather easy and it just requires a RCD FCU (Residual current device with fuse) and if the garage only has a single socket then I would not be too worried it would only get a code 2.

And BS7671:2008 542.2.5 The use, as an earth electrode, of the lead sheath or other metal covering of a cable shall be subject to all of the following conditions:
(i) Adequate precautions to prevent excessive deterioration by corrosion
(ii) The sheath or covering shall be in effective contact with Earth
(iii) The consent of the owner of the cable shall be obtained
(iv) Arrangements shall exist for the owner of the electrical installation to be warned of any proposed change to the cable which might affect its suitability as an earth electrode.

Which means you can use conduit as an earth.

What must be remembered if you ask someone to spend £20 for their own safety likely they will spend it. Ask them to spend £200 and they will start to think about it. Ask them to spend £2000 and answer in normally no. So to me “bongos” idea is best as it is likely to be followed. Second is “Chri5” as better device than what “bongos” suggested but costs more.

The “Can’t” attitude however correct does not help we must give positive help and have some consideration on cost.

 

[Gees]

Did the OP not say the conduit went underground ? Would that not be earthed then?

No, that would not count as being earthed.

Err !! large bit of metal buried in ground = NOT earthed ?
One Metre or so of copper plated steel hammered into same ground=Earth ?

 

[Paul_C]

If you can find a manufacturer to say that it's suitable for continuous immersion in water you might be OK, but as you probably can't then it is not suitable for that use and therefore it has been improperly installed.

There are galvanized conduits suitable for direct burial. Of course, we don't know what type of conduit is actually buried in this case, so it's all conditional upon the details.

Plus, is the conduit earthed?

Fair point, when I read the T&E was run in conduit I assumed that the conduit was also earthed - And it's not good to make such assumptions.

As for the fusing, in terms of overload protection, the design load for a socket can't really be anything other than what BS 1363 requires it to be able to support, which is 20A.

That 20A requirement in the current version of BS1363 isn't relevant to the design load for a single socket though, which will be 13A.

In terms of fault protection, did you do an adiabatic calculation?

Hang on, I thought we were talking about whether the installation complied with the current Regs. at the time of installation? That wasn't part of the Regs. in the 1970's. But obviously we can't know the prospective fault current without knowing the external loop impedance etc.

When the conduit was installed, it was not a requirement to earth it.

It was a requirement to earth it under the 14th edition Regs. in effect in the 1970's:

D.3 All metalwork of wiring systems (other than current-carrying parts), including cable sheaths and armour, conduit, ducts, trunking, boxes, and catenary wires, shall be connected to the appropriate earth-continuity conductors. This regulation does not apply to isolated metal parts referred to in Regulation D.9.

{.....}

D.9 Where isolation of metal is adopted for the prevention of dangerous earth-leakage currents, it shall be confined to the following items:

(i) Short isolated lengths of metal used for the mechanical protection of cables having a non-metallic sheath, other than overhead spans of conduit between buildings {.....}

I don't think you could argue that a 30 ft. run of buried conduit is a short, isolated section.

Wasn't there effectively a 0.75 derating at the 14th in '66 ( 'Coarse protection' tabulated ratings were allowed an increase by 1.33 when utilising 'close protection' )

Yes. Table 3M in the revised metric 14th edition 1970 shows a rating of 18A for 2.5 sq. mm T&E in conduit with coarse excess-current protection.

Err !! large bit of metal buried in ground = NOT earthed ?
One Metre or so of copper plated steel hammered into same ground=Earth ?

The conduit might be an effective earth electrode, but if it's not connected to the installation's earthing system, think about what would happen in the event of a fault on a TN-S or a TN-C-S system (or indeed a TT system with an old voltage-operated ELCB).

 

[Gees]

Err !! large bit of metal buried in ground = NOT earthed ?
One Metre or so of copper plated steel hammered into same ground=Earth ?

The conduit might be an effective earth electrode, but if it's not connected to the installation's earthing system, think about what would happen in the event of a fault on a TN-S or a TN-C-S system (or indeed a TT system with an old voltage-operated ELCB).

 

[securespark]

It could be argued it is short, compared to the overall length of the circuit cable.

 

[Atilla]

If you can find a manufacturer to say that it's suitable for continuous immersion in water you might be OK, but as you probably can't then it is not suitable for that use and therefore it has been improperly installed. Plus, is the conduit earthed?

foot tasting nice?

 

[ban-all-sheds]

If you can find a manufacturer to say that it's suitable for continuous immersion in water you might be OK, but as you probably can't then it is not suitable for that use and therefore it has been improperly installed.

There are galvanized conduits suitable for direct burial. Of course, we don't know what type of conduit is actually buried in this case, so it's all conditional upon the details.

I'm talking about the cable, not the conduit.

The conduit is too long not to have a join.

Therefore it will have a join.

You cannot guarantee that the join will be watertight throughout its life.

Therefore you cannot guarantee that the conduit will not be full of water.

Therefore you have to assume the worst case external influences, i.e. the cable will be continually immersed.

Therefore you have to use a cable which is suitable for continuous immersion.

foot tasting nice?

I'm talking about the cable:

The cable is 10m long, and improperly installed.

And again, while personally I'm not keen on T&E run in metallic conduit, there's nothing in the Regs. against it.

If you can find a manufacturer to say that it's suitable for continuous immersion in water you might be OK, but as you probably can't then it is not suitable for that use and therefore it has been improperly installed.

Do try and pay attention.

 

[ban-all-sheds]

That 20A requirement in the current version of BS1363 isn't relevant to the design load for a single socket though, which will be 13A.

The regulations pertaining to an unfused spur allow a twin socket outlet to be fitted, therefore an unfused spur has to be designed to support a 20A load.

Hang on, I thought we were talking about whether the installation complied with the current Regs. at the time of installation? That wasn't part of the Regs. in the 1970's.

Was it not?

In that case what did the regulations say in the 1970s about how to determine what level of fault protection was needed for a given cable?

Was a 30A BS 3036 acceptable for 2.5mm²?

But obviously we can't know the prospective fault current without knowing the external loop impedance etc.

We don't need to know that to work out the current in a L-N fault.

 

[Paul_C]

The regulations pertaining to an unfused spur allow a twin socket outlet to be fitted, therefore an unfused spur has to be designed to support a 20A load.

As a general principle, you don't have to select cable size based upon what somebody might fit on the end of it at some later date; you base it upon the design load as installed.

That said, however, when I got to thinking about this particular set up some more I've realized that it would not have been compliant even when installed, since the rating of the T&E in conduit would not meet the two-thirds rule.

In that case what did the regulations say in the 1970s about how to determine what level of fault protection was needed for a given cable?

It was based primarily on the fuse/breaker blowing/tripping on a certain proportion of overload of the rated value. I'll find the applicable Regs. from the 14th edition and post later.

Was a 30A BS 3036 acceptable for 2.5mm²?

In general, as a spur on a ring circuit, yes, subject to the two-thirds rule already noted.

We don't need to know that to work out the current in a L-N fault.

How would you calculate the fault current if you don't know the overall loop impedance?