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Topic Title: Wiring matters Summer 12
Topic Summary: Protective equipotential bonding
Created On: 15 June 2012 04:59 PM
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 15 June 2012 04:59 PM
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Avatar for Legh.
Legh

Posts: 3532
Joined: 17 December 2004

I have just read an article in the latest wiring matters magazine regarding protective equipotential bonding, page 29., and have a query.

I think the author may have missed the fact that touch voltage,Ut, is taken to be 50v as opposed to 230v.

Maybe I'm wrong, but I have been lead to believe that disconnection time, table 41.1 to be achieved then Zs <= Uo/Ia

but to minimize the touch voltage for the time the fault exists then
R = 50 / Ia

As an example, this is how I have interpreted it:

An 32A rated instantaneous shower with an earth fault will disconnect, if the circuit has been properly designed:

Zs <= 230 / 160 = 1.44 (1.15) ohms

but for the small amount of time the fault exists, 1/2 - 2 cycles of ac, the voltage might rise to, worst case value of 115 - 140v (depending upon the size of R1 and R2

Now applying 411.5.3 for TT systems R = 50 / 160 = 0.3 ohms

So the resistance of a combined R2 would need to be at least a sixth smaller than just using Uo.

What does the panel think?

Legh

-------------------------
Why do we need Vernier Calipers when we have container ships?

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"Science has overcome time and space. Well, Harvey has overcome not only time and space - but any objections."
 15 June 2012 05:24 PM
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AJJewsbury

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R = 50 / Ia

AFAIK that's just for RCDs - and in my opinion is there to prevent cumalative leakage current raising the potential at the MET (above local earth) to hazardous levels without the RCD tripping. (say you had a 500mA RCD, 499mA of leakage and a 200 Ohm rod - the MET would be at nearly 100V and no guarantee the RCD would trip.)

the voltage might rise to, worst case value of 115 - 140v (depending upon the size of R1 and R2

It's a TT example - so without bonding the voltage at the fault is a result of a potential divider with R1 on one side and Ra (i.e. R2+rod resistance) on the other - as Ra is likely to be "large" and R1 "small", the voltage at the fault will be fairly close to the full mains voltage.

(If they used a TN example, the benefit of bonding doesn't look anything so persuasive!)

- Andy.
 15 June 2012 05:34 PM
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davezawadi

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Hi Leigh
I'm a bit confused by your post, and don't have the wiring matters. You mention equipotential bonding at the top, and then seem to worry about circuit Zs. Equipotential bonding simply means that you cannot get a signfcant voltages between two accessible points under fault conditions, and therefore no shock. It is not expected that the equipotential bonding conductor carries the fault current, and therefore your calculation does not make sense. Say you bond the shower and the bath in a bathroom, a fault occurs on the shower whilst you are using it and touching the bath at the same time (some people have strange ways). The equipotential bonding conductor keeps the potential between the two fairly low because
1. it is short.
2. it carries only some (a small part generally) of the fault current.
Assuming disconnection by a fuse which s fairly slow, a shock is still not received. Without the bonding Zs as you calculated gives the shock voltage between the fault and any other earthed object in the installation, which as you say could be up to about 0.65 of the supply Uo.

-------------------------
David
CEng etc, don't ask, its a result not a question!
 15 June 2012 05:37 PM
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davezawadi

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Ah now I have seen Andy's post it is clearer, as the example is TT. Even so my comment is still the reason for the bonding, and it is still persuasive on the TNS example numbers I used. I will continue to fit it in some bathrooms because I don't find RCDs 100% reliable!

-------------------------
David
CEng etc, don't ask, its a result not a question!
 15 June 2012 05:39 PM
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gkenyon

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Legh,

What did you have in mind limiting Ut to 50 V?

Ut is only limited to 50 V under certain conditions (e.g. where disconnection times can't be achieved, as discussed by 411.3.2.6, then supplementary bonding can be provided in line with 415.2 and the object of this is to limit Ut to 50 V).

For TT systems where RCDs are used for protection in line with 411.5, limiting Ra*IΔn to 50 V doesn't mean Ut is limited to 50 V.

For example, consider we have Ra of 100 Ω, as per the article, and even a 500 mA RCD

Ra*IΔn = 50 V, and this satisfies the inequality Ra*IΔn ≤ 50 V.

However, the author's calculations of Ut with and without main bonding are still valid?

-------------------------
Eur Ing Graham Kenyon CEng MIET TechIOSH
 15 June 2012 05:40 PM
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gkenyon

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Andy, David, Seems we were all typing responses at the same time, along the same lines.

-------------------------
Eur Ing Graham Kenyon CEng MIET TechIOSH
 15 June 2012 05:49 PM
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davezawadi

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Ut is not limited in RCD protected installations but the contact time is (supposedly if it works). Ut is limited by R2/(R1+R2+Ze) in other installations but the contact time may not be very short. (0.4 or even 5 seconds). With lots of equipotential bonding R2 may be lowered to any touch voltage you fancy.

-------------------------
David
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 15 June 2012 07:53 PM
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sparkingchip

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 16 June 2012 05:09 PM
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Avatar for Legh.
Legh

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Apologies for the delayed response, I've been out partying.......

Thanks gents for the info. The points noted are:

1/ Zs and Bonding do different things. While I agree with this there will be a particular time when an earth fault exists that both R2 of the faulted circuit and the main protective bonding and associated supplementary bonding will enhance the R2 with further parallel paths

2/ I used 50V as the maximum safe voltage allowable (although maybe fatal to some).

3/ I accept the necessity for protective bonding, full isolation and RCD protection in TT systems.as the voltage potential at the MET could rise to mains potential

but just to add a little more,

where bathrooms and swimming pools are fed from TN systems, the time that a fault exist will still produce a rise in potential however short the time period is. Protective bonding ensures that the potential differences on metalwork stays low but also decreases R2.

I'd always thought the purpose of R <= 50/Ia or IAN was to ensure that the voltage at any point would never be any higher than 50V.

I now see that there is no reference in BS7671 to the use of this rule accept for TT systems and as long as the disconnection times are met then Zs <= Uo/Ia is sufficient to provide protection.

So I assume that where a bathroom or swimming pool is not covered by RCDs then the protection relies on the touch voltage being held below 50V and the circuit disconnection times of the faulted circuit.?


Legh

-------------------------
Why do we need Vernier Calipers when we have container ships?

http://www.leghrichardson.co.uk

"Science has overcome time and space. Well, Harvey has overcome not only time and space - but any objections."
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