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Topic Title: Tripping of breakers to prevent involuntary muscle contraction
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Created On: 01 May 2013 09:27 PM
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 01 May 2013 09:27 PM
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adammid

Posts: 81
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Does anyone know how fast a fuse or circuit breaker must disconnect if someone is receiving a shock and cannot let go?

At approx 13-15mA it causes involuntary muscle contraction and may prevent someone from letting go.
 02 May 2013 12:00 AM
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ebee

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it will not disconnect.
an RCD might do (10mA type)

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Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 02 May 2013 12:10 AM
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SKElectrical

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Originally posted by: adammid

Does anyone know how fast a fuse or circuit breaker must disconnect if someone is receiving a shock and cannot let go?
.


"must disconnect"

I'm afraid it's not that simple.
Do you mean : how long is it before the shock causes fatality? Because again, that varies on a variety of factors.

If you mean : how long does it take before fuse / circuit breaker disconnects under fault conditions? Then I'm sorry to sound like a parrot but it's not that simple.

Fuses and breakers are not designed specifically to disconnect under all kinds of fault that you have described? In fact they alone probably wouldn't disconnect the supply.

Maybe you mean: with added RCD protection? Generally they are tested to operate under 200milliseconds at 30mA. And must operate under 40ms at 5mA (I think!).
Look here http://www.google.co.uk/url?sa...bvm=bv.45921128,d.d2k

Older installations used ELCBs which were like today's RCD but had different operating characteristics which are probably unknown as testing never incorporated this check.
 02 May 2013 06:06 AM
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ebee

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"Maybe you mean: with added RCD protection? Generally they are tested to operate under 200milliseconds at 30mA. And must operate under 40ms at 5mA (I think!).
Look here http://www.google.co.uk/url?sa...bvm=bv.45921128,d.d2k "

SK,
your figures are a bit wrong I`m afraid.

With the exception of deliberate time delays.
An RCD must trip at 100% rated residual current within 200ms to our old BS or within 300ms to the EN standard.
If intended for personal protection (10 & 30mA say) then at 5 times rated residual current must trip within 40ms (150mA for a 30mA RCD).

This makes our old BS appear safer/quicker than the EN in theory.
In my experience most RCDs of either type trip similarly and well within the trip times of both standards in practice. Others might disagree.

-------------------------
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Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 02 May 2013 10:19 AM
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jcm256

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adammid
You should ask yourself why I ask this question. What reaction do you want to arouse. At the back of your mind, you will know that a fuse or circuit breakers offer no protection.

An RCD of 10 to 30mA may help to protect against electric shock where current flows through a person from a phase to earth. It cannot protect against electric shock where current flows through a person from phase to neutral or phase to phase, for example, where a finger touches both live and neutral contacts in a light fitting; a device cannot differentiate between current flows through an intended load, from flow through a person
 02 May 2013 10:52 AM
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AJJewsbury

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In the 'usual' case of earthed metal being made live by a fault, for 230V systems and most (<=32A) final circuits, the regulations usually require disconnection in 0.4s where the voltage at the fault is around half mains voltage (TN systems) and 0.2s where it might be close to full mains voltage (TT systems). That doesn't limit the amount of current flowing through the victim (which is likely to be low hundreds of milliamps), (so it'll still hurt, cause muscle spasms etc), but it's unlikely to be directly fatal - at least for about 95% of the population.

If the victim is directly touching a live (line) conductor, then as others have correctly said, fuses and (overcurrent) circuit breakers offer no practical protection - hence the importance of both insulation & earthing (backed up by 30mA RCDs in some situations).

- Andy.
 02 May 2013 10:53 AM
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Legh

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I must say that I get a little 'zipped up' by what lets what through and how much damage gets done within the timescales indicated by various protective devices.
An oscillogram of a BS 88 device appears to disconnect within around 5ms when the fault current is a short circuit. That would suggest if you were a parallel path to that fault you would receive a proportion of the fault current and a proportion of the voltage as it dipped to zero.
If, however, you were the fault, ie, You became connected between phase and earth, then the device is unlikely to operate and you would receive as much current as your body resistance would allow at the full voltage.
If this were the case when a RCD was in the circuit then, and this is the bit which still confuses me, you would get the current at the supply voltage to whatever the RCD rating was rated at, which would be in the process of disconnecting within 2 cycles of AC current (150mA max. for a 30mA rated device.

Legh

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 02 May 2013 11:46 AM
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AJJewsbury

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If this were the case when a RCD was in the circuit then, and this is the bit which still confuses me, you would get the current at the supply voltage to whatever the RCD rating was rated at, which would be in the process of disconnecting within 2 cycles of AC current (150mA max. for a 30mA rated device.

Nope, you'd get the full current depending on the voltage & your body resistance - the RCD itself wouldn't limit the current at all while its contacts were closed. The RCD's rating only decides whether it'll trip or not (and maybe how fast), but it can't of itself limit the current (to 150mA or anything else) until its contacts open (at which point the current should be zero) (hopefully there won't be too much arcing involved with the resistance of the human body limiting the current to a few hundred milliamps).
- Andy.
 02 May 2013 12:54 PM
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daveparry1

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Rcd's don't limit current at all, (until they open of course!) they just limit the time that the current can flow for,

Dave.
 02 May 2013 01:24 PM
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Legh

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Originally posted by: AJJewsbury

If this were the case when a RCD was in the circuit then, and this is the bit which still confuses me, you would get the current at the supply voltage to whatever the RCD rating was rated at, which would be in the process of disconnecting within 2 cycles of AC current (150mA max. for a 30mA rated device.


Nope, you'd get the full current depending on the voltage & your body resistance - the RCD itself wouldn't limit the current at all while its contacts were closed. The RCD's rating only decides whether it'll trip or not (and maybe how fast), but it can't of itself limit the current (to 150mA or anything else) until its contacts open (at which point the current should be zero) (hopefully there won't be too much arcing involved with the resistance of the human body limiting the current to a few hundred milliamps).

- Andy.


I'm sure we had this same debate about 5 years ago and the answers receieved went part way but then stopped without a logical picture being presented, well in my mind anyway.

Now, a device that is designed to interrupt the supply current at a given point , in this case an imbalance between its contra-wound phase and neutral windings, will operate the trip coil at that set current.? So a fault current at its given rating will cause the device to operate at, and up to, a maximum given time, apply more current due to a lower circuit resistance and more current will flow but again will be limited by the time it takes to operate the trip.

I agree, under fault conditions you will see a full fault voltage, but, in my mind, you will not see the full possible fault current because the device will limit at the point of its rating and beyond up to a maximum determined by the speed of operation.

You you say that was more or less correct ?

Legh

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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."
 02 May 2013 01:34 PM
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OMS

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agree, under fault conditions you will see a full fault voltage, but, in my mind, you will not see the full possible fault current because the device will limit at the point of its rating and beyond up to a maximum determined by the speed of operation.


Assuming we are talking about indirect contact faults here, you won't see the full mains voltage - you'll experience a touch voltage - which will be lower, and often much lower than mains voltage.

If you accept that the Touch Voltage Vt = If x R2 then the equation contains no refernce to In of any type of protective device.

Current limitation is simply a function of time of operation - if you can open a device in less than one half cycle, there will be a diminution in the prospective fault current due to the arc voltage across the opening contacts and the point of opening biased aloing the voltage wave (ie at what part of the cycle)

regards

OMS

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Failure is always an option
 02 May 2013 02:07 PM
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AJJewsbury

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Now, a device that is designed to interrupt the supply current at a given point , in this case an imbalance between its contra-wound phase and neutral windings, will operate the trip coil at that set current.? So a fault current at its given rating will cause the device to operate at, and up to, a maximum given time, apply more current due to a lower circuit resistance and more current will flow but again will be limited by the time it takes to operate the trip.

I agree, under fault conditions you will see a full fault voltage, but, in my mind, you will not see the full possible fault current because the device will limit at the point of its rating and beyond up to a maximum determined by the speed of operation.

I must confess I'm not entirely following.

Are you suggesting that the shock current will gradually increase from zero to a maximum (dictated by the overall loop impedance) over time, so the quicker the RCD opens the lower the maximum current?

I know with very large fault currents that's the case to some extent (due to collapsing magnetic fields in the supply transformers etc?) , but where we're only talking about a couple of hundred milliamps (due to the resistance of the victim's body), I would have guessed that the network could supply that sort of current pretty much instantly - especially if the fault happened to occur at a peak of the a.c. cycle.

- Andy.
 02 May 2013 02:51 PM
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OMS

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I suggest the OP clarifies if this is direct or indirect contact shock

regards

OMS

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Failure is always an option
 02 May 2013 03:52 PM
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Zs

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Your are asking some curious questions adammid,

http://www.theiet.org/forums/f...d=52328&enterthread=y

Are you a lawyer?

Zs
 02 May 2013 11:14 PM
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sparkingchip

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The protection required is basic insulation so that you don't get a electric shock in the first place.

A fuse or circuit breaker will not save a life if someone is getting electrocuted.

Earthing, bonding and a RCD may save a life by keeping a electric shock below lethal limits, generally considered to be 50 milliamps of current at 50 volts for 50 milliseconds.

Andy
 03 May 2013 08:10 AM
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Zs

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here you go Adammid,

from an HSE document entitled 'Electrical Safety for Entertainers' which was sent to me recently. Word for word just in case you are a lawyer.

1mA perception level
2mA Muscles convulse
30mA Trip rate for RCD protection because anything above this level is dangerous
50mA Respiration is affected, victim dies of suffocation
100mA Ventricular fibrillation, usually fatal
250mA Current necessary to light a 60 Watt lamp is sufficient to electrocute five people simultaneously.

Zs
 03 May 2013 08:55 AM
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daveparry1

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I'm amazed i'm still around after reading that Zs! Having spent the first 40 years of my working life as a TV engineer where nearly all fault-finding work had to be done whilst the set was working getting a belt was a fairly regular occurence! Since changing over to electrical installation a few years ago I very rarely get a shock these days though,

Dave.
 03 May 2013 09:11 AM
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AJJewsbury

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Most standards for protecting against electric shock are based on a set of time/current curves published by the IEC: e.g. http://www.electrical-installa...nwiki/File:FigF01.jpg

- Andy.
 03 May 2013 09:23 AM
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OMS

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OK - lots of you will have come acros that old phrase -

"It's the volts that jolt, it's the mills that kill"

You can die (easily) with a few tens of milliamps - and much less if the vicitms response is diminished by illness, alchohol, drugs etc - but you need a voltage to drive that through the resistance and it needs to endure for sufficient duration.

To put into contrast the milliamp values routinely quoted, IEC would suggest that about 95% of a population will survive a full direct contact 240V shock if the exposure time doesn't exceed 0.16 seconds.

This is the upper end of a curve that would show that at 50V no disconnection time is required - ie you could survive it indefinately.

Given that we use insulation, barriers, enclosures, placing out of reach for protection from direct contact, then it's indirect contact we are worried about.

Indirect contact is goverened by the touch voltage Vt = If x R2. With bonding in place and setting Ze to zero (which can't happen) and taking a view on the relative CSA's of line and CPC conductors) that will show an asymptotic voltage for Vt of around 140Volts. In parctice it may well be lower or higher depending on what ratio of line to CPC is selected by the designer (it's really only the UK that uses reduced cross section CPC's in T&E cable)

from IEC curve 2 - we can survive that for about 0.4 seconds - seem like a familiar number ?

Regards

OMS

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Failure is always an option
 03 May 2013 09:35 AM
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adammid

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Originally posted by: AJJewsbury

Most standards for protecting against electric shock are based on a set of time/current curves published by the IEC: e.g. http://www.electrical-installa...File:FigF01.jpg



- Andy.


Andy

Thats what I was looking for, many thanks. From the graph, line 'b' shows at approx 15mA (the point of involuntary muscle contraction, preventing someone from letting go) a breaker should trip in 1000ms (1 second). Am i correct in thinking this?
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