Armoured core cable Size

[Andy I 2013]

Hi every one

what size armoured core cable for my new to be observator will i need ? 4mm,6mm,etc

[red dwalf]

If it's not too far from the house 2.5mm should be more than enough.

[Davey-T]

2.5 will be plenty, get a sparks to make off the ends properly and advise on earthing.

Dave

[Ouroboros]

May I clarify what your 2.5mm refers to? Is that 2.5mm^2 per core? In other words something like this https://www.tlc-direct.co.uk/Products/SW2dot5slash3.html

[Davey-T]

May I clarify what your 2.5mm refers to? Is that 2.5mm^2 per core? In other words something like this https://www.tlc-direct.co.uk/Products/SW2dot5slash3.html

Yes, that's the stuff.

Dave

[Ouroboros]

Thanks.

[Andy I 2013]

If it's not too far from the house 2.5mm should be more than enough.

its about 40 meters away

[Pan Euro]

It will be fine. I ran mine in 2.4 and it's about the same distance away. It's been like that for two years and no issues.

[Davey-T]

There appear to be two schools of thought on the earthing question, either earthing the steel sheath and having an RCD at the main consumer unit or having a ground earth spike at the obsy end.

Dave

[Andy I 2013]

 Thanks a lot pan  i am hope to have my odservator up and runing this summer coming

[Starlight 1]

Work out the Amps on every thing being uses+   meters away will give you the size of cabe for the load .

[101nut]

I believe, if you stick figures into the regs and assume that you're looking at a 20A radial circuit, you need 4mm cable for a run of that length due to voltage drop.

Earthing is important. With that length I'd be inclined to earth the armour at the consumer unit then, at the observatory, isolate the armour and install an earth stake. But you really need to talk to a sparky about that as the entire installation should be installed and tested according to current regulations.

When it comes to electrics, "works", "safe" and "reliable" are not mutually exclusive ...

AndyG 

[Davey-T]

My garage is 40 meters from the main board and is wired in 2.5 SWA cable, I can run a 1hp compressor off it with no problems, no cable warming or the garage lights dimming.

Dave

[JamesF]

To get the earthing right requires that you know how the house is connected to the grid which is not always obvious.  If you don't get it right then people can end up getting nasty surprises (for example, you're not supposed to go creating a path to ground when someone working on the supply wouldn't expect one).  Having a sparky involved makes a lot of sense.

James

[Mick J]

The link above is 3 core cable so the earth is easy,  I would have thought linking the armoured sheath was a good idea, the glands come with earth tags even if the box's are plastic.  Sparky will know and test final install.

[JamesF]

The link above is 3 core cable so the earth is easy

I'm not an electrician, but my understanding is that's not actually true.

In some cases it's ok to "export" the house earth to an outbuilding.  In other cases you absolutely shouldn't, but should instead terminate the earth inside the box without connecting it to the outbuilding earth (though it should be connected at the house end).  Earth in the outbuilding then has to be provided via a grounding stake, but that has to meet certain electrical standards in terms of impedance (or something similar) to be considered a reliable earth and it shouldn't be possible to short it to the house earth.  One of the factors that decides which is safe and which isn't is how the earth is provided to the property from the grid because there are several different ways it can be done and my understanding is that doing the wrong thing can leave a supposedly dead circuit with the potential (hah!  geddit?) to become live.

I have to admit that I always thought it was simple until I started doing some research to check the safety of some existing wiring that we have to outbuildings.  That was when I discovered an entire can of worms   If there are any SGL posters who are sparkies by trade they'd doubtless be able to explain the risks rather better.

James

[JamesF]

Bearing in mind my previous comment about not being an electrician...

Some mains supplies provide an earth that is actually the same as the neutral.  They're just split from each other somewhere on the supplier side -- perhaps at some substation or other local distribution unit.  In that situation I think if you add a new earth and there is a neutral fault after the place where the supplier earth is split from the neutral, your new earth can end up becoming the return path for everything on your side of the fault, including other properties.  At best that's a fire hazard.

On the other hand, if you export your house earth, particularly if it's any distance, the resistance of the earth conductor can stop RCDs working correctly in the event of a fault.  Again if it's any distance, the earth potential of your outbuilding can be different to that of the main house and your body can make a nice bridge between the two but may not take kindly to doing so.  I think that if you do this you have to bond absolutely everything conductive to the exported earth.

There are three common earthing systems in use in the UK, called TN-C, TN-C-S and TT.  They have different requirements regarding earthing.  I can't recall which is which of the first two, but I know TT is where you have your own earth rod because that's what we have.

James

[richyrich_one]

If you haven't bought the cable yet, this was the cheapest I could find. 

http://www.superlecdirect.com/cables-and-accessories-c1/bs5467-steel-wire-armoured-cables-s6/p-xlpe3x2.5-3core-25mm-swa-bs5467-steel-wire-armoured-cable-harmonised-cores/

[teifistar]

Bearing in mind my previous comment about not being an electrician...

Some mains supplies provide an earth that is actually the same as the neutral.  They're just split from each other somewhere on the supplier side -- perhaps at some substation or other local distribution unit.  In that situation I think if you add a new earth and there is a neutral fault after the place where the supplier earth is split from the neutral, your new earth can end up becoming the return path for everything on your side of the fault, including other properties.  At best that's a fire hazard.

On the other hand, if you export your house earth, particularly if it's any distance, the resistance of the earth conductor can stop RCDs working correctly in the event of a fault.  Again if it's any distance, the earth potential of your outbuilding can be different to that of the main house and your body can make a nice bridge between the two but may not take kindly to doing so.  I think that if you do this you have to bond absolutely everything conductive to the exported earth.

There are three common earthing systems in use in the UK, called TN-C, TN-C-S and TT.  They have different requirements regarding earthing.  I can't recall which is which of the first two, but I know TT is where you have your own earth rod because that's what we have.

James

An RCD does not rely on an earth connection, it operates when there is an imbalance in the live and neutral conductors. This imbalance is caused by a current flowing to earth through the persons body and not returning through the neutral conductor, to operate the RCD this current is very small 30mA typically and the disconnection time is very fast so no harm comes to the person touching the conductors.

This is why you are advised always to use an RCD when using electrical equipment outdoors e.g. hedgecutters. It doesn't matter what your earthing arrangments are the RCD will protect you. Remember you only get one chance.

Peter.

PS try Screwfix for your cable.

[JamesF]

It may be that I meant MCB rather than RCD.  I honestly can't recall.  I'm just sure that I've read there's some safety issue with protection devices possibly becoming unreliable if the earth resistance is too high.

James

[JamesF]

A quick hunt on the interwebs suggests that RCD may be correct.  The issue appears not to be that the RCD won't function, but that if the earth has too high a resistance then the RCD might not trip sufficiently quickly to protect someone in the event of a live-to-earth fault.

James

[Mick J]

I am not an electrician either, we run less than 10 meters with this sort of cable and terminate in a Builders supply box that contains an RCD,  I like to think all shed & obs circuits are protected.   

[teifistar]

A quick hunt on the interwebs suggests that RCD may be correct.  The issue appears not to be that the RCD won't function, but that if the earth has too high a resistance then the RCD might not trip sufficiently quickly to protect someone in the event of a live-to-earth fault.

James

As I said RCD's operate independently of your earth arrangments or the integrity of your earth. That is why they are recommended. See this link for an explanation.

http://www.explainthatstuff.com/howrcdswork.html

The RCD will trip when the residual current (current flowing to earth through the persons body or anything else which has become live) reaches the stated trip current. The speed at which it trips is  dependent on i.e.e. electricity regulations and inherent in the design of the RCD.

When running a cable to a shed, garage etc. The house end should be connected to the supply using an RCD of appropriate size and sensitivity, for protection and to be able to isolate the supply/

HTH.

Peter

[Gina]

Correct me if I'm wrong but AFAIK MCB means Miniature Circuit Breaker or you could call ia a Maximum Current Breaker in that it actuales if the maximum current is exceeded.  It has nothing to do with earth leakage.  RCD = Residual Current Device and measures the difference in current between Line and Neutral - it is NOT connected to the earth line and doesn't rely on an earth connection.  When something like a person causes current to flow from line/live to earth it unbalances the LN currents and will trip the breaker when it exceeds the low level of just 30mA - insufficient to cause human harm from electrocution.  It used to be called an Earth Leakage Breaker (ELB).  Leakage can also be caused by dampness causing conductivity between line/live and earth and thus a potential hazard.

[teifistar]

Correct me if I'm wrong but AFAIK MCB means Miniature Circuit Breaker or you could call ia a Maximum Current Breaker in that it actuales if the maximum current is exceeded.  It has nothing to do with earth leakage.  RCD = Residual Current Device and measures the difference in current between Line and Neutral - it is NOT connected to the earth line and doesn't rely on an earth connection.  When something like a person causes current to flow from line/live to earth it unbalances the LN currents and will trip the breaker when it exceeds the low level of just 30mA - insufficient to cause human harm from electrocution.  It used to be called an Earth Leakage Breaker (ELB).  Leakage can also be caused by dampness causing conductivity between line/live and earth and thus a potential hazard.

Correct, MCB's replace fuses and are very much more accurate, i.e. a 13 amp fuse may take considerably more than 13amps to blow and have a relatively slow disconnection time. A 20amp mcb will trip at exactly 20 amps and very quickly.

RCD's and ELB's are different. RCD's rely on an imbalance in the currents in the live and neutral conductors  to trip the breaker. There is a potential scenario when an RCD wouldn't trip. If a person was to hold both live and neutral conductors and be well insulated from earth, very thick soles etc. then no current would pass to earth and there would be no imbalance in the live and neutral and so RCD would not trip. An ELB is an old device which relied on the current travelling to earth to trip the breaker.