Hello all

I am wondering if someone could please answer a question for me. I am trying to understand why 0.4s is the required disconnection time for a circuit breaker? I have searched high and low for the information but everywhere just states that it is the requirment under AS3000. Why not have 0.2s or 0.5s?

I also understand that as a fault current occurs the frequency drops at this point. So as the frequency stretches out the fault current required for an instant trip is obviously stretched over time. By having a 0.4s trip time it allows for the current to get to a point to cause an instant trip at 7.5times a c type breaker. Otherwise you would always get nuisance tripping. Is this the only reason?

The answer lies in DD IEC/TS60479-1 "Effect of current on human beings and livestock"

A simple explanation is along the lines of: 0.4 seconds is roughly the maximum time that 95% of people should be exposed in a left hand to feet shock for a voltage of around 240 V a.c., with a low risk of irreversible physiological side effects, given average "dry" impedances of the human body, operating in a "TN" system.

(In BS7671, 0.8 seconds is permitted for circuits up to 120 V a.c. for similar conditions from a TN system.)

RCDs operate more quickly, reducing the risk of any physiological effects further (corresponding to a different part of the chart).

Hence, RCDs are a good solution for additional protecton where people are likely to handle the powered appliances and the magnitude of shock voltages is theoretically up to Uo, and mcb's/fuses selected to operate with 0.4 s are a good solution for protection of a general purpose circuit in a fixed TN-type installation in general where additional protection in the form of main bonding is in place.

The disconnection time of 5 s assumes that the shock voltage is likely to be lower, and/or people are less likely to be in contact with the circuit in a fault (e.g. a distribution circuit, where the final circuits are otherwise protected by 0.4 s disconnection times).

The good thing for us, is that other people have already done all the hard work, and we simply need to address the correct disconnection time for the particular circumstances from our national wiring codes, e.g. BS7671, AS3000, National Electrical Code, etc. - many of which are based on and indeed contribute to IEC60364-series.

Lastly, with mcb's, in the EU (and I would assume Australia too), with our agreed nominal voltage being 230 V (240V), the standard for the mcb's, EN60898, is set for 0.4 seconds because of this, so if you use an EU mcb for a 120 V circuit, you're still having to design to the "0.4 seconds" disconnection time (any lesser fault current gives a disconnection time of > 5 seconds as the thermal element will be dictating the disconnection, not the magnetic element).

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Eur Ing Graham Kenyon CEng MIET

Edited: 15 June 2012 at 08:35 AM by gkenyon

.4sec for a TNCS and TNC, then on a TT system we can fit a 100mA time delay RCD for circuits not requiring a 30mA RCD


Consider some statement from HSE publications, for general use:

An RCD required to minimise the risk of personal injury should have a rated tripping current of no more than 30 mA and should not have an adjustable time delay. Although the 30 mA versions are often used, those with lower rated tripping currents (typically, 10 mA or below) are readily available and may be used to provide additional protection where nuisance tripping is not a problem.
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(Of course adjustable time delay would only apply to large industrial applications where for circuits protected by MCCBs up to 125A, where under short-circuit conditions the energy let through by the MCCB is too great for the RCD, which could suffer catastrophic failure). But where would .4sec sit with adjustable time delay, would it comply.

Consider statement
In addition, employers are required under regulation 3 of the Management of Health and Safety at Work Regulations 1999 to assess the risks to the health and safety of their employees while they are at work, in order to identify and implement the necessary precautions to ensure safety.

Would changing, (TT Supply) up front 80amp TP& N 100mA RCD, to a 100mA time delay RCD for workshops and stores breach the above statement. (Still thinking about that but the 100ma none time delay which I am told is tripping frequently). The premises is something like below>
http://www.hse.gov.uk/press/2010/coi-wm-10510.htm

Consider statement:

The table below summarises the effect of different strengths of electric current on the body.
Current
(mA)
Length of Time Likely Effects
0-1 Not critical Threshold of feeling; undetected by person.

5-6 Not critical Tingling sensation; threshold of perception

7-15 Not critical Threshold of cramp; independent loosening of
the hands no longer possible. 16-30 Minutes Cramp like pulling together of arms; breathing
difficult; limit of tolerance.

30-60 & above
Prolonged exposure can be fatal Death can occur in a fraction of a second
31-50 Seconds to minutes Strong cramp like effects; loss of consciousness
due to restricted breathing. Longer exposure may
lead to fibrillation
51-500 Less than one heart period (750ms) No fibrillation; strong shock effects.
Over 501 Less than one heart period. Fibrillation; loss of consciousness; burn marks.

Where does .4sec sit with the above statement?


jcm

As posted above, 0.4 seconds is a reasonable compromise between what the typical person can withstand, and keeping the costs and complexities reasonable.
0.4 seconds is also the worst case, in a great many installations, the actual trip time will be far quicker.
And in many cases the actual shock voltage will be about half the mains voltage, and therefore less of a risk.

The frequency wont drop noticeably during a fault in a consumers installatiom, provided it is connected to a reasonably sized grid system.

Remember that in most small and medium sized installations, that fault currents will be tiny compared to the total capacity of the grid system.

In the case of a large fault on a very small grid system (such as might be found on a remote island) then the frequency might drop noticeably.
Like in an installation connected to a generator, the frequency will drop, perhaps noticeably during a fault.

It makes more sense if you look at the graph (rather than generalised table): http://www.electrical-installa...enwiki/Electric_shock

If you take the resistance of the human body as 1000 Ohms, then you can read the bottom scale as touch voltage rather than milliamps. - i.e. for 120V (the likely touch voltage in a 240V TN system) 400ms is just on the safe side of the C1 curve - for 240V (i.e. what you might be exposed to in a TT system with no or ineffective bonding) - it's closer to 200ms.

- Andy.

Thanks that reply makes sense but difficult.
Had cause to look up the meaning of words "Imperceptible" and "Perceptible".
The best I can pick with appropriate meanings is below. As they have many meanings.


All the same, I suppose if an unauthorised person dabbles in machinery control panels and gets a severe shock or worse, although in a TT installation (or other systems) a 100mA RCD would not give personal protection it could be brought up in court, fitted a what: a time delay RCD making the situation worse. The only hesitation was you are changing the designer's installation original design.


imperceptible meaning(s) impossible or difficult to perceive by the mind or senses
perceptible - capable of being perceived by the mind or senses; "a perceptible limp"; "easily perceptible sounds"; "perceptible changes in behavior"

Thanks
jcm

That's about right - in this context: "Imperceptible - too small to be felt - no feeling. Perceptible - can be felt (might be a slight tingle, might be a huge belt!)" is how I think about it.

- Andy.