How much current safe for 30m extension?

[Graham W]

I doubt that a *you* of any value would survive if it were to pass the fusing current of the plug-top fuse!

[John Rumm]

I guess I was kind of hinting (gently) that the term PSC implies a L-N short, rather than L-E.

Hence the fact that it is TT (from an earthing arrangement point of view) will make no difference, although to be fair the likelihood of a TT setup being out "in the sticks" a long way from a substation is greater, and hence _may_ have a higher supply impedance as well.

;-) (yes painfully... having just spent a fortune on a big pile of CUs and RCDs ready for the great CU replacement party....)

[John Rumm]

Depends on if you are in serise or parallel with it!

[ehsjr]

A ~ 2 volt drop at 4 amps means ~ 1/2 ohms. You had 800 feet of wire - 400 out and 400 back. That means your extension cord wire had to be rated at (1000/800) * 1/2 = .625 ohms per 1000 feet. Your extension cords had to be made from #6 or #8, given those numbers. That is highly unlikely.

A typical heavy duty extension cord 100 feet long would most likely use #14 wire. #14 wire is rated at 3.1 ohms per 1000 feet. At 800 feet, if the 4 amp load was running, there would be a drop of about 9.9 volts. If #12 wire (rated at 2 ohms per 1000 feet) were used in the cords, the drop would be about 6.4 volts. The ohms per 1000 feet numbers come from table 9 in the National Electrical Code.

There has to be an error in this, somewhere.

Ed

[Darcy B]

What is earth fault loop impedance?

(Yup, I have tried Google!)

[Sammo]

I am the original poster with two 30m extension cables. You refer to RCD protection.

I have got a plug-in RCD device but I know nothing about earth fault loops. It is similar to the one in this illustration.

should my RCD device be plugged in to best avoid the problem of earth fault loops:

(1) At the mains supply end where the first extension goes into the mains socket. (2) In the middle of the two 30m extensions. (3) At the far end of the two extensions where the appliciance I am using is plugged in?

[Sammo]

The cable reel has two ratings. I took the unwound rating to get

10A/2400W.

Ha! You have spotted my lack of knowledge about these things - like current carrying capacity. I wasn't too sure how these things are determined.

However, if the cable has resistence then presumably current capacity (ignoring voltage) gets reduced. I am thinking of W = I^2 * R.

I posted just now and asked about RCDs (see copy below). If you have some info on using an RCD then I would be pleased to hear.

Thanks, Sammo.

==========

MESSAGE ID =

I am the original poster with two 30m extension cables. You refer to RCD protection.

I have got a plug-in RCD device but I know nothing about earth fault loops. It is similar to the one in this illustration.

should my RCD device be plugged in to best avoid the problem of earth fault loops:

(1) At the mains supply end where the first extension goes into the mains socket.

(2) In the middle of the two 30m extensions.

(3) At the far end of the two extensions where the appliciance I am using is plugged in?

[Sammo]

What a pity they didn't list the cross-sectional area (calculated from the diameter) as it seems that many people refer to cross sectional area when determining current capacity.

[Sammo]

CU?

[Ian Stirling]

Consumer Unit. The box where the electricity comes into the house, which contains RCD/breakers/fuses for the individual circuits.

[Andrew Chesters]

= Consumer unit. The RCD should protect the whole of the "outdoor" section.

Earth Loop Fault Current = the current that would flow in the event of a live to earth fault. This current is calculated from the earth loop impedance (measured or calculated) including suppliers cable. This value is then compared with standard graphs for various protective devices (fuse, circuit breakers) to ascertain the disconnect time. If this does not meet the requirements, additional methods of protection are required. This could be changing to a faster breaker, and/or adding earth leakage protection.

[John Rumm]

It is the sum total of the resistances af all the wiring in the path to earth or ground. It is significant because should a major fault occur (like a wire falls inside an appliance and shorts to the casework, or you cut through an extension lead), the earth fault loop impedance will place a limit on the maximum current that can flow to earth.

Ideally this fault current wants to be large, so that it causes the protective device (fuse / breaker) to open quickly. We have regulations in the UK that require a circuit with socket outlets to disconnect in under 0.4 secs in these situations.

The fault loop impedance is also dictated by the impedance of the earth provided at the supply where it comes into the house. In the UK there are three common ways the power company can provide the supply - with two of them (known as TN-C and TN-C-S (aka PME)) the supplier provides a good earth (i.e. typically well under 1 ohm) which when used with suitable fusing and circuit breakers should result in good disconnection times in most cases. However if you add long circuits with undersized earth conductors (i.e. big extension lead for example!), then the impedance creeps up and lowers the fault current that could lead to much greater electrocution risk. The third type of supply common over here does not supply an earth at all (this is typical for power fed via overhead wires - typically into rural locations), and relies on a local earth rod that is staked into the ground. These tend to give much higher resistance earths and hence you can no longer rely on them to allow enough fault current to flow, so the whole installation must be protected by additional Residual Current Device circuit breakers (RCDs) to detect any leakage from the circuit and cut off the power that way. (RCDs are also mandatory here for all socket circuits that may potentialy feed portable equipment that could be used outside).

There is some more info here:

[John Rumm]

fault loop impedance. Also see here for more background:

Where should my RCD device be plugged in to best avoid the problem of

No - but better than 3

no - but better than not at all!

Having the RCD at the start of the cable will mean the whole cable is protected by it. So should you accidentally do the "hedge trimmer through the extension lead" exercise, the RCD hopefully will cut off power to the lead, rather than leaving live exposed conductors flopping about in the wet grass under your ladder!

[Dimitrios Tzortzakakis]

cable is not that expensive, it's copper after all.I can see in an invoice that I paid 28 cents a meter for a 3G1.5 mm^2 A05VV-U cable.A 2.5 mm^2 conductor, single core, which we generally use to connect washing machines, costs 9 cents a meter .Or 6 mm^2, for ranges, 22 cents a meter.Why don't you ask an electrician to construct you an extra heavy duty extension cable, with industrial-grade plugs?I did in college, when doing my Practicum, for a high-temperature oven, three-phase, 25A, which plug tended to overheat.Cost was negligent.

-- Tzortzakakis Dimitri?s major in electrical engineering, freelance electrician FH von Iraklion-Kreta, freiberuflicher Elektriker dimtzort AT otenet DOT gr ? "Peter A Forbes" ?????? ??? ?????? news: [e-mail]...

[The Natural Philosopher]

You can draw the same current through any length of wire, asd long as it isn't coiled up, because the heat per unit length does not vary with the length, and that is what causes temp rise.

Which is why cable is rated in amps, not watts. Nor yet amp-meters.

The v drop on even quite extended lengths of extsnion cable is not likely to be an issue.

[Owain]

"John Rumm" wrote | Depends on if you are in serise or parallel with it!

I *still* get confused with blue being live because it's a 'brighter' colour than brown in cables; WTF did cerise come into it? Will Channel 5 be giving us Colin & Justin's How Not To Write Wiring Regulations in the near future...

Owain

(This posting may contain UK-specific cultural references.)

[Owain]

"Sammo" wrote | However, if the cable has resistence then presumably current capacity | (ignoring voltage) gets reduced. I am thinking of W = I^2 * R.

No, because that resistance - and hence the heating effect - is spread along the length of the cable. If 1m of cable is rated for 10A then 100m of cable will also be rated for 10A. You will probably need a thicker cable to reduce the voltage drop, but that is not the same as the current-carrying capacity of the cable.

| I have got a plug-in RCD device but I know nothing about earth fault | loops. It is similar to the one in this illustration. |

| Where should my RCD device be plugged in to best avoid the problem of | earth fault loops: | (1) At the mains supply end where the first extension goes into the | mains socket.

Here, because the RCD protection must be applied to the whole extension flex, because it is in effect a portable appliance.

| (2) In the middle of the two 30m extensions.

No, because that would leave the first flex (or more specifically - someone handling it) unprotected.

| (3) At the far end of the two extensions where the appliciance I am | using is plugged in?

No, same reason.

If you were wiring a *permanent circuit* then RCD protection could usually be applied at the load end, because fixed wiring does not need such a high standard of protection.

Note, however, that using an RCD plug on only one extension lead raises the possibility that someone might plug the extension leads togther the wrong way round, with the result that the first flex would be unprotected. The way round this is to have an RCD plug on both leads -- this will do no harm, but either or both may trip in the event of a fault -- or use an RCD socket.

The point remains -- why are you contemplating using 60m extension cables?

Owain

[Watson A.Name - "Watt Sun, th]

overheat.Cost

Negligent! Hah-hah-hah-hah!

[Sammo]

I live in a flat and my car is kept in a garage in a block which is in the yard. The garages do not have power and sometimes I might need to use a power drill, electric light, soldering iron, charge a run-down battery, etc.

This is the layout:

I'm on the third storey and my cable flex would be slung along the building for about 30 metres and be supported once where it goes out of my flat and supported again (at about the same height) where it leaves the building 30m later.

The next 15 metres of cable would be a sort of descent to my garage roof. The last 5 metres or so takes the power to where I want it. (I am using 30m + 30m because those are the lengths on my two extension reels).

In this scenario, I don't really need to protect the cable from accidental cutting or damage anywhere along its length as it runs along the building or in the air away from any likely harm.

For my *own* protection as a user of an applicance at the far end of the cable it seems that it might be better to put an RCD close to whatever appliance I am using. Is this correct?

OTOH maybe a domestic RCD is so sensitive that it is likely to work perfectly well at the mains end even when I am chiefly looking to protect me at the far end?

[Palindr☻me]

Basically, the gizmo senses when the current being shoved up the live wire is greater than what comes back down the neutral wire. If it is greater, then the difference, instead of coming back down the wire, is going somewhere else - like through /you/. If it finds a difference it cuts off the power, very quickly.

So its sensitivitity will be the same no matter where you put it - as the same current should be going up the live and back the neutral - where ever you measure it.

Putting it in the wall socket ensures that the insulation of the whole length of leads as well as the load is being monitored. which give the most protection. Plugging it in after the leads means that the unit doesn't monitor for faults in the leads themselves, only in the load.

The only thing you gain from plugging it in at the load end is if you have a piece of hardware that is prone to tripping rcds on switch-on. Having it near the load makes it easier to reset the thing if it trips and saves the hike back upstairs. But you would only plug it in there for that sort of reason.

Always use the test button each time you set this up. They are remarkably sensitive devices that have to sense a difference of a few hundredth's of an ampere in a load of many amperes. For the cost, as I think another op may have said, why not fit two - one at each end? Particularly if you are lying on fairly damp ground under a car, clutching a mains lamp in your hand.. I have a main one protecting the sockets for the whole house (excepting a ring main that only feeds my fridges and freezers - where each socket has an rcd), but still use an additional plug-in one when doing anything that makes me nervous.