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Topic Title: Regulation 411.3.2 - why differing maximum disconnection times?
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Created On: 05 December 2017 12:09 PM
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 05 December 2017 12:09 PM
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jonel

Posts: 11
Joined: 19 October 2017

Hi All,
Can anybody kindly explain why for low voltage final circuits NOT exceeding 32A the time of disconnection for a TT system is 0.2s and for a TN system it is 0.4s.

Also, for low voltage final circuits exceeding 32A the disconnection time increases - for TN it is a massive 5s and for TT it is 1s.

I am trying to understand why the times are actually different. They are all the same voltage and I feel that in the TT/TN case it is related to the Zs value.
But is is the jump for circuits exceeding 32A that I really struggle with. I can see the increased to time to allow discrimination from circuits downstream. The 'Electricians Guide' states that the longest disconnection times for protective devices, leading to the longest shock times and the greatest danger will be associated with lower levels of fault current. I don't appreciate that this is the case so I would be grateful for any advice on that (there is also large diiference of 1s for TT and 5s for TN.

Thanks

Jonel
 05 December 2017 12:56 PM
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AJJewsbury

Posts: 16095
Joined: 13 August 2003

The difference between 0.4s and 0.2s for TN and TT is due to the difference in voltage the victim is exposed to during the fault. On TN systems the protective conductor is about the same impedance as the line conductor, so the supply 230V is divided about equally between them - meaning the victim is exposed to around 115V. On TT systems - due to the much higher impedance of the earth side of the loop (having two lots of soil around electrodes to contend with) the voltage at the point of the fault can be much higher - often not far below 230V. (Have a look at the famous graph of the effects of electric shock on the human body from IEC 60479-1 - if you presume a body resistance of 1k Ohms, you can read volts for milliamps).

The allowance for longer disconnection times for circuits over 32A and distribution circuits is just practicality. The greatest risk from shock comes from small hand-held appliances - and these are almost always found on smaller sized circuits. (In previous versions of the regulations, 5s was allowed for all fixed equipment, 0.4s being needed only for hand-held equipment (or sockets that might supply such) and for circuits outdoors. 5s itself again is really just down to tradition and practicality - if you size circuit conductors for a reasonable voltage drop the resulting maximum loop impedance generally means that a suitable fuse will blow within about 5s.

The statement " longest disconnection times for protective devices, leading to the longest shock times and the greatest danger will be associated with lower levels of fault current." is generally true of overcurrent protective devices - e.g. fuses and MCBs - generally the lower the earth loop impedance, the higher the earth fault current, the quicker the device opens. (Although with MCBs operating in the 'instantaneous' region (i.e. under magnetic rather then thermal operation) there comes a point where the mechanical bits move as fast as they can and larger fault currents don't produce an further decrease in opening time). With RCDs the effect of larger fault currents is minimal.

- Andy.
 05 December 2017 01:38 PM
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mapj1

Posts: 9689
Joined: 22 July 2004

Agreed, plus the expectation is that as part of normal design, there is always some current to earth. Perhaps 0.1% or so of the circuit rating to pull a typical figure from the air - so a 6A lighting circuit leaking a few mA is normal, and a 30ma RCD or RCBO will be fine. You may expect rather more for a socket circuit, proportional to the number of sockets served .

A submain feeding ten such lighting circuits, cannot reasonably be protected by an RCD of the same rating, and be reliable but equally that is not the trip that needs to operate when a light fitting or a switch is smashed, and it is easier to arrange mechanical protection or to use armoured cables, for the larger sub-mains. You may argue about the choice of 32A as the breakpoint, as it seems to be driven by 'assume 0.1%' rather than anything more magical, but it works in practice.
So circuits 32A and up are not protected to a level that is designed to save the life of someone touching it, as a person touching it is not seen as the likely cause of fault, but rather to prevent a fire, and to remove a shock hazard on an unattended system after a few seconds.

It leaves 45A shower and cooker circuits, which are high current final circuits, as an anomalous case.

-------------------------
regards Mike
 05 December 2017 01:58 PM
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tattyinengland

Posts: 994
Joined: 23 November 2006

This below thread may cover further questions you may have on disconnection times- a natural sort of carry on to your first post.

http://www.theiet.org/forums/f...=81647&highlight_key=y
 05 December 2017 10:41 PM
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jonel

Posts: 11
Joined: 19 October 2017

Well, what a thorough and most excellent reply. I am very grateful for the time taken in this response. It is a lot clearer to me now.

Thank you very much

Jonel
 06 December 2017 08:07 AM
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GordonB1

Posts: 3
Joined: 02 December 2017

I always thought that the disconnection times were to designed to protect the cables and insulalation from over heating under large fault currents. If the touch voltage on an extraneous conductive part is taken as 230 V and a body resistance 1K Ohms, then Ohms law applies and the victim will get 0.23 Amps flowing through their body. This is enough to cause death or serious injury, but not enough to disconnect any over current device. This is why 30 mA RCDs are employed as additional protection.
 06 December 2017 09:11 AM
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mapj1

Posts: 9689
Joined: 22 July 2004

And for the enormous over currents ,hundreds of times the cable steady state rating, that would do damage to the wiring on a sub-second basis, the ADS covers that. And it is such large faults where the old adiabatic (= no time for heat loss) approximation is most valid.
There are some very good demonstrations of cable being over run by what seems like an unaccetable margin, a good few times the normal rating, and taking many seconds to get hot enough to be a problem.
this one by John Ward is a well balanced explanation, in terms of proper rigorr and not just being ooh-ah as some other folk are.

-------------------------
regards Mike
 06 December 2017 12:18 PM
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AJJewsbury

Posts: 16095
Joined: 13 August 2003

I always thought that the disconnection times were to designed to protect the cables and insulalation from over heating under large fault currents. If the touch voltage on an extraneous conductive part is taken as 230 V and a body resistance 1K Ohms, then Ohms law applies and the victim will get 0.23 Amps flowing through their body. This is enough to cause death or serious injury, but not enough to disconnect any over current device. This is why 30 mA RCDs are employed as additional protection.

Not exactly. Traditionally overcurrent protective device generally both protect the conductors and provide protection from electric shock (in the case of faults, or indirect contact as it used to be known). If you look at some of the tables for max Zs for fuses for different sized c.p.c.s in the OSG or GN 3 you'll see that lower values (or NP) are needed for some smaller conductors before the values "plateau out" for larger sizes - that's because the value needed for ADS doesn't protect the smaller conductors - and faster disconnection times are required.

Provided exposed-conductive-parts are properly earthed, overcurrent protective devices can usually provide adequate protection against electric shock without having to resort to RCDs - the critical point is that 230mA or whatever is tolerable by the human body provided it's only for a short duration. For small final circuits on TN systems both fuses and MCBs are usually quite capable of disconnection well within the required time. Please don't go assuming that circuits without 30mA RCDs have no protection from electric shock.

Where RCDs come into their own is where the earth fault current can't flow down the protective conductor - e.g. because someone has picked up severed flex or has drilled through a hidden cable and is in contact just the line conductor - completing the circuit via the general mass of the earth of some other earthed part. Only a relatively few situations require such additional protection however.

RCDs are also very useful where Zs is too high for overcurrent protective devices to disconnect in the required time - not normally the case in TN systems but can occasionally occur (especially with long extension leads), and are of course the preferred means of ADS in TT systems - but again don't need to be 30mA devices to provide adequate protection against electric shock under fault conditions (indirect contact) - 100mA, 300mA or 500mA devices can be quite acceptable.

- Andy.
 06 December 2017 03:17 PM
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gkenyon

Posts: 4981
Joined: 06 May 2002

Originally posted by: AJJewsbury

The difference between 0.4s and 0.2s for TN and TT is due to the difference in voltage the victim is exposed to during the fault. On TN systems the protective conductor is about the same impedance as the line conductor, so the supply 230V is divided about equally between them - meaning the victim is exposed to around 115V. On TT systems - due to the much higher impedance of the earth side of the loop (having two lots of soil around electrodes to contend with) the voltage at the point of the fault can be much higher - often not far below 230V. (Have a look at the famous graph of the effects of electric shock on the human body from IEC 60479-1 - if you presume a body resistance of 1k Ohms, you can read volts for milliamps).
Yes ...

Just a thought those, we need to note it's not all "victims" - inside a well bonded building (I won't try and define that), touch voltages from TT are actually much lower then TN-C-S / TN-S, but if you run an extension outside, and are standing on the "general mass of earth" rather than something at a potential close to the MET, then you are the "victim" who receives close to a supply-voltage shock.

-------------------------
EUR ING Graham Kenyon CEng MIET TechIOSH
G Kenyon Technology Ltd

Web-Site: www.gkenyontech.com
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