ECN Forum Forums General Topics of the Trade Non-US Electrical Systems & Trades Is the 0.4 seconds requirement enough?

I have touched this subject before, but this time I'll start a separate thread.

In many countries there is a requirement that the circuit is matched with a protective device so that a short between earth and live is cleared within 0.4 seconds (5 seconds in some cases, but that doesn't matter with MCB's)

If there isn't an RCD, this is achieved by limiting (by design) the earth fault impedance so that the breaker trips in case of a fault.

Here comes my concern: This is a far as I know calculated for the outlet, which is fine for a cooker or something that remains in place. But for a general purpose socket, the connected cord and possible extension cord can add significantly to the impedance.

(This does not apply to BS 1363 sockets since these are fused, but well to industrial type sockets.)

If I plug in 10 metres of 1.00 mm2 cord, which isn't unrealistic, I add an impedance of 0.34 ohm. How does that relate to the total impedance?

IIRC, the impedance for a TN-C service can be taken as 0.3 ohms, which should correspond roughly to a 40A supply. We allow a maximum voltage drop of 4% in the branch circuit. If the branch circuit is 20A radial with equally sized conductors (T&E cable is worse), this corresponds to a impedance of 0.46 ohms. This leaves us with a total impedance to socket of 0.76 ohms.

The resulting current is then 230V/0.76 > 300 A. This will trip a typ C breaker.

But if we add the cord, the impedance rises to 1.1 ohm and the current falls to just over 200A. Still sufficient, but only just.

Had we used a longer cord or a thinner cord, we would be below the limit.

The reasoning works for short circuits line to neutral too, where the danger is 'just' fire.

As I understand it, the 0.4 second requirement was calculated on the basis of the effect of an electric shock on a person using an appliance which develops a fault.

If we have a TN-C-S system with equal-sized circuit and protective conductors, then a direct line-to-earth short on the appliance will result in a touch potential of one half the supply voltage.

It's a long while since I read the details, but I think it's something to do with there being a greater chance of the heart going into fibrillation if the shock is sustained for longer than the period of one heartbeat at the shock voltage of around 120V -- All based on the average current likely to flow and the average person's physiology, of course.

It's a good point about the reduced-size ground wire in British T&E cable, as that could raise the touch potential considerably at the end of a long run. I'd like to see the ground wire specified to be at least the same CSA as the circuit conductors.

No matter what you plug into the socket outlet ,longer & or thinner wire,etc.. It is outside the scope of the regs , BS7671.

So is the 0.4s disconnection time enough ? well yes provided that the fixed wiring is ok & equipment plugged in satisfies the appropriate standards, and that you don`t daisychain too many extension leads

About a year ago i was on a farm visiting a friend who worked there, and he was welding some gates from an extension lead made up of a 100m drum of 2.5mm T+E wound round a garden hose reel with wheels on i was surprised that the welder could strike an arc.

I can buy an 25 m extension lead and plug in whatever in it. I might not need that long an extension lead, but that happens to be what I have.

What you do "after" the socket is indeed outside the scope of the wiring regs, but nevertheless the wiring is intended to be used. Otherwise you could have a situation where 'the wiring is safe as long as you don't use it'.

What I was getting at was really that an impedance from a typical cord should be included in the calculation. Let's say 15m of 1.0 mm, which gives 0.5 ohm.

230V mains power will generally maintain a usable voltage even over a long narrow run like that though.

You should see the problems you get with relatively short runs of 24V cable for garden lighting... voltage drops all over the place.

Personally with gardening equipment the best sollution is to install proper blue Ceeform sockets around the garden on posts fed with properly rated, properly burried sheilded cable run inside heavy conduit.

Overhere it's normal practice to fuse them at 13 amps and have a local RCD fitted.

That minimises the use of ridiculously long extension cords and ensures good protection as well as removing water permiable domestic plugs from the garden.

In a british/irish climate it's really insane to continue to use electrical equipment in potentially wet grass without proper connectors and proper RCDs etc..

Although, I do have an elderly relative who used BS546 15 amp trailing sockets and plugs in the garden on a regular basis big old round black ones from the 40s!

[This message has been edited by djk (edited 07-28-2003).]

djk said

"Although, I do have an elderly relative who used BS546 15 amp trailing sockets and plugs in the garden on a regular basis big old round black ones from the 40s!"

His longevity coupled with his regularity appears to indicate a safe system!