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Topic Title: Rcds in series
Topic Summary: Tncs to TT
Created On: 14 August 2014 10:40 AM
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 14 August 2014 10:40 AM
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tillie

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Hi , I have a very embarrassing question to ask and I am quite ashamed that I do not know or that I have forgotten ( I dont know which is worse ).

This is a hypothetical question.

If I was running a supply down for a garden shed consumer unit and the supply was taken from a 30ma protected way in the house consumer unit and the shed was converted to TT would I need a 30ma rcd in the shed consumer unit ?

Assume that the Shed has no ECPs so no bonding required.

Help please.
 14 August 2014 10:54 AM
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hifly

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no

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 14 August 2014 11:26 AM
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AJJewsbury

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Physically a single RCD will operate perfectly satisfactorily (discrimination etc notwithstanding).

Regs wise, it's a bit cloudy I think. 411.5.1 seems to say that every exposed-conductive-part protected by a single device shall be connected to the same earth electrode. I've yet to make any sense of that requirement in the context of modern RCDs (it would make sense for old voltage-operated ELCBs of course). There is an argument that 411.5.1 only applies to the TT portion of the installation, not the TN part, so it's debatable whether the TN part being connected to a different earth is a contravention or not (or if you go down that path whether not having a protective device within the TT part is a contravention of 411.5.2). Certainly if the house was already TT, the reg would seem to say you couldn't have a separate rod for the shed and share an RCD - which makes doing so for a TN house look a bit dubious.

I suspect it's a case of the regs were worded in the good old days before PME when you didn't normally have mixed earthing systems within a single installation and haven't kept up with developments. But given the words as they are currently printed... it's all as clear as mud.

- Andy.
 14 August 2014 05:32 PM
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Zs

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Originally posted by: tillie

and the shed was converted to TT would I need a 30ma rcd in the shed consumer unit ?



Assume that the Shed has no ECPs so no bonding required.



Help please.


Well, at risk of making myself look like a dunce....Hifly, how can you say no to that? I don't follow.

Tillie, I'm assuming the example house is a TNCS because you've said 'converted' and that in this example the earth from the house, down the garden has been broken on arrival at the shed.

No bonding from the shed end (isn't that at a football ground?).

So, the new TT at the shed diverts fault currents down the new electrode, not back to the house.

Without an RCD how would ADS be achieved?
A TT must be protected by an RCD (generally).

So I say Yes, you would need a 30mA RCD in the shed.

I'll look in later to see if I've mis read an op again. Still at work right now.

Zs
 14 August 2014 05:41 PM
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AJJewsbury

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Zs - I think you've missed the upstream 30mA RCD in the house. (RCDs just look at 'missing current' they don't care where it's gone to).
- Andy.
 14 August 2014 05:50 PM
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Zs

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Indeed Andy, However, with a broken CPC, where is it going to or to exhibit some semantics, how much is flowing in it?

With the means of earthing broken at the shed you are assuming that limiting Zs at the shed circuits is 'low' err, like very.

Zs
 14 August 2014 05:57 PM
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daveparry1

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I still think it should have an rcd at the far end!
 14 August 2014 06:15 PM
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Zoomup

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I would say that the shed TT supply would have to have its own 30 mA RCD and an earth rod at the shed position. All TT supplies would have to have an RCD and an earth rod to allow fault current to flow to earth rather than through a person from a live part in case of fault. I would prefer to have a separate supply (non RCD) from the house, that is one not running throught any house RCD because an external fault would knock out some house circuits in case of shed/garden fault. I recently came across a shed supply run in armoured cable from a house ring circuit. For ease of installation the shed supply had been connected very neatly from the nearest house socket via a FCU. The house had a "17th Edition" split load RCD protected consumer unit. The shed also had a small garage consumer unit with a 30 mA RCD. A mains pond pump had become faulty. (water ingress). The pump had a insulation resistance of about 0.2 Meg Ohm L to E. When first switched on the shed RCD tripped off. A second attempt caused the house RCD to trip off, disconnecting all of the kitchen sockets and fridge and freezer. Keep outside supplies separate to the house circuits I say. Outside supplies are more likely to trip off because of water ingress or general conditions or abuse.

Bye,

Z.
 14 August 2014 06:18 PM
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Parsley

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If there's no extraneous-conductive-parts do you really need to TT the shed?

Regards
 14 August 2014 06:27 PM
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jcm256

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Don't think series is quite the right word to use in your heading, it should be cascade.

The RCDs may not act in the order as below:

Series
Similar things placed in order or happening one after another.



cascade (plural cascades)

A series of electrical (or other types of) components, the output of any one being connected to the input of the next.
 14 August 2014 06:28 PM
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Zoomup

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Originally posted by: AJJewsbury

Zs - I think you've missed the upstream 30mA RCD in the house. (RCDs just look at 'missing current' they don't care where it's gone to).

- Andy.


Yes RCDs do look for an imbalance and do not care where the fault current goes to, but, it would be nicer if the earth leakage current went down to earth via an earth rod and not to earth via a person who touches a live piece of equipment under fault conditions. The earth rod route for the fault (L to E) current would automatically disconnect the supply before a person could touch a live piece of equipment that is live and just standing there waiting for somebody to touch it, thus tripping the RCD.

Bye,

Z.
 14 August 2014 06:38 PM
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Zoomup

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Originally posted by: jcm256

Don't think series is quite the right word to use in your heading, it should be cascade.



The RCDs may not act in the order as below:



Series

Similar things placed in order or happening one after another.







cascade (plural cascades)



A series of electrical (or other types of) components, the output of any one being connected to the input of the next.


Whatever you call the arrangement of a supply running through two RCDs, if both are rated at 30mA unless one is of a time delayed type, it is hit or miss as to which will operate first under fault conditions. , leaving the other still in the on position. You don't want a shed fault knocking out half of the house electrics. A faulty garden pond pump or light can trip off an RCD if plugged in to a single pole switched 13 Amp socket even if the socket switch is in the OFF position. So if you were away on holiday and the shed supplied pond pump fills with water in the motor compartment creating a N to E fault, it could trip off the house RCD of the circuit supplying it. Fridge and freezer then do not work, or in winter the background heating no longer works in the house to keep the frost out. Keep house and garden circuits separate I say.

Bye,

Z.
 14 August 2014 06:48 PM
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daveparry1

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I always think parallel is a better description in these cases?
 14 August 2014 06:53 PM
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Zoomup

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Originally posted by: Parsley

If there's no extraneous-conductive-parts do you really need to TT the shed?



Regards


Perhaps because someone may use a class 1 piece of equipment on the lawn using an extension lead from the shed. True (lawn) earth and a CEN PME earth on the metal casing of the equipment may be very different and a shock risk may present itself if a bare footed user touched the earthed case whilst standing on the lawn if there is a rise of potential on the CEN neutral.

Bye,

Z.

Edit. Correction. I am getting my classes mixed up sorry. I should have said Class 1 for a piece of portable electrical equipment with a conducting metal casing/enclosure, sorry.

Edited: 15 August 2014 at 10:19 AM by Zoomup
 14 August 2014 07:33 PM
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hifly

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as has been said above, if an imbalance is seen the RCD will trip, so in my view there is no benefit having a 30mA rcd in the shed as well,

however this is not how i would do it, i would want to see either no rcd protection to the submain if possible, taking account of all the other factors we don't have information on or a 100mA s type at the house and a 30mA in the shed.

the OP did not ask us to design it for him he asked if he should have 2 x 30mA rcd's in series I don't see any point in that. Perhaps i should of elaborated somewhat but i thought i would pop in a quick answer before i went out.

i am always happy to be corrected if I have something wrong, it won't be the 1st time and it won't be the last time either.

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 14 August 2014 07:59 PM
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Parsley

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Originally posted by: Zoomup

Originally posted by: Parsley



If there's no extraneous-conductive-parts do you really need to TT the shed?







Regards




Perhaps because someone may use a class 2 piece of equipment on the lawn using an extension lead from the shed. True (lawn) earth and a CEN PME earth on the metal casing of the equipment may be very different and a shock risk may present itself if a bare footed user touched the earthed case whilst standing on the lawn if there is a rise of potential on the CEN neutral.

I'm well aware of the difference between true earth and PME earth. Not convinced by your example should every ground floor socket should be TT? and class 2 equipment doesn't have an earth connection does it?


Bye,



Z.


Regards
 14 August 2014 09:15 PM
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tillie

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Hi , thanks for all replies,

I am glad






Hi, thanks for all replies. I am glad


that it was not as straightforward as I thought it might be..

An old work colleague phoned me and asked me the question and I was ashamed that I did not know for sure.

He has taken a supply from the house consumer unit and ran it in swa 30mtrs down a garden and installed another consumer unit in the shed.

The sway supply has been taken of of a dual rcd board with 30ma protection.

He has made the shed a TT supply and installed an earth rod.

He asked me does he need to install another rcd at the shed and I was not 100% sure.

This is now troubling me so I need to clear this up for my own piece of mind.

I know that ideally the sub main should be time delayed but that is not the case here.

He has inherited the 30ma rcd at the house and there is nothing he can do about it.

My immediate view was that the second rcd was not needed but the fact that he has converted the shed to TT has me troubled.

If I saw a TT consumer unit without an rcd I would initially think it was wrong but if the Zs at the furthest outlets in the shed were within limits then surely the house rcd would deal with it.

I also think that the two rcds would both operate at the same time anyway.

Advice please

Still thinking

Regards
 14 August 2014 09:16 PM
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AJJewsbury

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Indeed Andy, However, with a broken CPC, where is it going to or to exhibit some semantics, how much is flowing in it?

With the means of earthing broken at the shed you are assuming that limiting Zs at the shed circuits is 'low' err, like very.

Sorry, I'm not following. Do you mean an accidentally broken c.p.c. leaving a section of the the installation unearthed or the deliberate break separating the TN system from the TT one?
If the former, then ADS is disabled regardless of whether it's TT or TN, RCD or no (your only hope then is supplementary protection). If the latter, then the earth route is via the shed electrode just like it would be on any TT system (Ra < 200 Ohms or whatever) - there's still the 1st RCD on the supply side. The lack of a 2nd RCD doesn't change that. Tillie didn't suggest omitting the TT earthing system - just the 2nd RCD. Or I'm missing something obvious (again).


For sure, it would be better to separate ADS for the shed from the rest of the installation (even have all RCBOs rather than share a couple of RCCBs across many circuits), but that's not what the question was about (I think).

I always think parallel is a better description in these cases?

To me, two RCDs in parallel would have their inputs connected together and their outputs connected together (like two cables connected in parallel). I like the 'cascaded' term though.

- Andy.
 14 August 2014 09:21 PM
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Zs

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Ok, clever people must look aside. Ordinary sparkies are allowed to look. Bear of little brain here is about to attempt an explanation and as you know, I'm not very good at this. But seeing as I've got the best mentor in the universe I owe you this to have a go because I've got drawings of stick men in sheds in my notebook. OMS, please don't be too harsh but you are allowed to comment when you see this. So long as you support of course .

Now I am doing a visual of OMS with his head in his hands ' what is she up to?'...

Andy's response. I can't argue with it but I don't approve all that much. Well, I can't argue if we're dealing with a perfectly intact and perfectly functional TN installation at the house end and a cable to the shed end of perfect integrity. And not if we're brilliant at physics and wear nice clothes to work. In the engineer's world, a designed system will always be as designed you see.

So it is all perfect up to the shed. Then, after that lovely TN and the perfect cable down the garden to the shed, there'd be a break in the CPC ready for the TT for the shed. I'm trusting you to agree with me so far.

A TT. in our sparky language, indicates that some poor bloke or blokess has hammered in a rod aka an earth electrode but that isn't really what TT means, as I am sure you know.

So, this shed is made of wood with a wooden floor, like sheds have in real life. It is a lovely hot summer and the wood is tinder-dry.

Andy is in his shed messing around with things electrical on a hot day when a fault occurs. He hasn't got anything on his feet. In fact, it is so hot that (for impedance reasons you understand) he is wearing only a pair of Speedo Budgie Smugglers. So he is worth about 1000 Ohms.

The floor of the shed, as dry as it is, is worth about 22,000 Ohms.

Connected back to the house and the general mass of earth that's 23,000 Ohms.

Andy is fiddling with the light switch and there is a fault on the lighting circuit and it goes right though him in an attempt to get to ground.

Ohms law says that I =V/R so, I=230/23,000

So I= 0.001 Ohms which is 10mA.

I think that's about the level of your heart going a bit wobbly.

What is more, that 10mA going through Andy is not going to cause any kind of disconnection. It is going to stay at 10mA for as long as it wants.

Now, if there is an earth Electrode on this set up and it is at 200 Ohms, then do the sums again and you'll see that enough current is going to flow to trip the RCD. So everything is ok.

Until the gardener comes round one day, and puts his shears through the ugly green and yellow cable which is right in the way of the thriving Phaseolus Vulgaris. And we often talk about how our Green and yellows get cut through.

Or until the RCD in the house sticks. Until the integrity of the Line and neutral back into the RCD in the house is compromised. Until there is a supply neutral or impedance problem.

And so on.

Deep breath. I've done it now and I hope that you agree.

RCD at the shed end every time ( I looked it up, The shed end does exist, it's at Cheslea FC).

Zs
 14 August 2014 09:26 PM
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Zs

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Andy, just seen yours.

Broken deliberately, for the TT, but see above.

I think on this one there are two points of view and I hope that neither are incorrect.

Zs
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