Irony

[says-les]

We go to length by design to ensure shock protection. Zs must be measured and be below a certain figure.
If the user is working at the end of a 30 Metre extension, he is risking a shock if a fault to earth occurs while using an earthed metal tool. The disconnection time will exceed 0.4 sec. No? I am assuming that Zs at the tool will exceed the max allowed value in the BBB.

 

[Murdoch]

An extension lead , like any appliance plugged in, isn’t our problem

 

[Geoff1946]

That's why long extension leads should always be on an RCD.
The RCD will detect the earth current and disconnect as specified whilst an over-current device would depend on the cable length.

 

[ProDave]

The BIG irony is we go to great lengths to stop someone poking a tiny object through a spare hole in a consumer unit and getting a shock etc, but we still fit BC and ES lamp holders that you can remove the lamp from and put your finger in and touch the live contacts.

If we were really serious about protecting people from touching live parts those sorts of lamp would have been outlawed years ago and they would all use something akin to the GU10 base where there are no easy to touch live parts with the lamp removed.

 

[Sharpend]

You can get fittings that have retractable shields built in. Remember fitting them years ago, might have been Hager or deta possibly?

 

[Andy™]

the extension will have a fuse in the plug so Zs at socket may still be within limits of that. doesn't apply to 60309 plugs though...

 

[binky]

some lamp holders have an interlock that leaves the contacts dead without a lamp in it.

 

[UNG]

Crabtree's safety lampholder has been around for about 40 years I remember them being specced on a council housing job back in the early 80's when they first came to market

 

[ProDave]

Crabtree's safety lampholder has been around for about 40 years I remember them being specced on a council housing job back in the early 80's when they first came to market

So if the EIT were remotely concerned with electrical safety, they would have been a requirement since about the 16th edition? If so plenty more people would make them and they would now be normal.

 

[SPECIAL LOCATION]

We go to length by design to ensure shock protection. Zs must be measured and be below a certain figure.
If the user is working at the end of a 30 Metre extension, he is risking a shock if a fault to earth occurs while using an earthed metal tool. The disconnection time will exceed 0.4 sec. No? I am assuming that Zs at the tool will exceed the max allowed value in the BBB.

As Andy mentioned there will be an additional protective device in the plug-top... { such as a BS1362 fuse. }

Have you checked your Zs calculations taking into account ALL of the protective devices in your example??

i.e. I assume if you add a small CU in a garage you would take account the garage protective devices,
not just the sub-main supply device back at the main CU?

I suspect some of your assumptions could well be incorrect?

 

[Mike P]

So, on a long extension lead where would you put the RCD, at the source to protect the lead or at the socket to protect the person?

 

[binky]

So, on a long extension lead where would you put the RCD, at the source to protect the lead or at the socket to protect the person?

ideally at the plug, the number of people I've encountered who have cut cables in gardens is quite shocking! Bad pun intended

 

[ProDave]

So, on a long extension lead where would you put the RCD, at the source to protect the lead or at the socket to protect the person?

If the socket it is fed from is RCD protected I would not have one on the extension lead.

 

[Mike P]

My point really is. With a long extension lead and the Zs being well out of its spec at the socket, would the RCD at the house operate under fault conditions if the person using it managed to get himself between phase and earth? If he had an RCD in the extension socket at the end of the lead, would that not offer “him” better protection?

 

[BorisJ]

My point really is. With a long extension lead and the Zs being well out of its spec at the socket, would the RCD at the house operate under fault conditions if the person using it managed to get himself between phase and earth? If he had an RCD in the extension socket at the end of the lead, would that not offer “him” better protection?

No. The same current would pass through both RCDs. Either of both will trip.

 

[Mike P]

No. The same current would pass through both RCDs. Either of both will trip.

The earth return impedance must be lower at the far end, if the person is the grounding element/conductor

 

[Geoff1946]

The earth return impedance must be lower at the far end, if the person is the grounding element/conductor

That's irrelevant. It's the current which is being detected.

 

[SPECIAL LOCATION]

The earth return impedance must be lower at the far end, if the person is the grounding element/conductor

Don't forget RCD's detect an imbalance between the live conductors...
(For a 30ma device, not below 16ma but definitely above 29ma)...

And.... If there is insufficient current flowing to cause an RCD to trip...
Then there will be insufficient current flowing to endanger any people, property or livestock..

So skip back to series & parallel electrical physics...

'Loads in parallel'.. divide the current..
i.e. Voltage is the constant across all of the parallel circuits connected to a CU..
but the currents vary according to the various circuit loads....

Whereas, 'Loads in series'.. divide the voltage..
i.e. Current is the constant from source to the furthest end of the circuit.
so pretty much the same current imbalance at either end of an extension lead!

[side note:- Wiring regulations require us to take into consideration 'Voltage Drop' on longer circuits, not 'Current Drop' !]

RCD's of a similar spec, in series anywhere along the length of a circuit, from 'source', 'middle', or 'end of a circuit',
will have negligible difference in operating times..

 

[says-les]

The BIG irony is we go to great lengths to stop someone poking a tiny object through a spare hole in a consumer unit and getting a shock etc, but we still fit BC and ES lamp holders that you can remove the lamp from and put your finger in and touch the live contacts.

If we were really serious about protecting people from touching live parts those sorts of lamp would have been outlawed years ago and they would all use something akin to the GU10 base where there are no easy to touch live parts with the lamp removed.

Good point

 

[Mike P]

Many Thanks “Trailor Boy”. Well put