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Discuss Body’s resistance with mcb only in the Electrical Courses and Electrical NVQ's area at ElectriciansForums.net

Wilson12

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If there’s 230v and the body’s resistance is high ..
Meaning the current will be very low but enough to kill you.
Can we get electrocuted without having a 30ma rcd to stop this as the 6amp breaker will not have tripped ?
 
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We did some very crude calculations on this sort of thing at college

Assuming dry skin , no wet floor etc etc

Iirc a b curve mcb would not trip if a person grabbed the live wire
 

Wilson12

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I don’t understand why rcds are additional protection then because loads of places just have mcbs which don’t protect people
 

telectrix

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I don’t understand why rcds are additional protection then because loads of places just have mcbs which don’t protect people
MCBs provide overload and fault protection. RCDs protect from silly fingers directly contacting parts which are live.
 

Richard Burns

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The effects of electric shock are very variable and subject to a whole range of different considerations.

The resistance (or impedance) of the body is one consideration but then you have to say where the current travels, hand to foot, foot to foot, hand to hand, head to hand and so on. The resistance varies widely.

How good a contact is there to the source of supply?
How wide a contact is there to the source of supply?
How wet is the body and with pure water or salt water?

On a very basic level the hand to hand resistance of the human body in dry conditions with a wide contact area is generally taken to be about 1000Ω.
This could be the equivalent of holding onto an earthed railing and putting the other hand around a live cable.
Using I =V/R this would give you a current through the body of 230mA.
No circuit breaker (0.5A minimum for 60947-2) will ever disconnect at this level.
Therefore you are then looking at the period of experiencing the shock, based solely on how long contact is maintained, to decide on the effects of the shock.
Above 200ms there would start to be the chance of irreversible effects including death from heart fibrillation.

If the body is wet with water containing salts dissolved in it (say sea water) then the resistance may be halved.
Similarly the resistance from hand to foot may be three quarters.

If you experienced a 500mA shock in this way you would be immediately into the chance of electrocution by heart fibrillation.

The use of circuit breakers is only there to prevent cables overheating and burning, it is not designed to protect people from electric shock. Though it does have the effect off ensuring the faulty circuit is off when someone starts looking at it.

The only protection for people from electric shock is chance and rubber soled shoes!

That is why RCDs are so important for safety.
 

Wilson12

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  • #10
Thanks that clears it up I know rcd haven’t been around as long as overload but

It seems crazy then that there’s soo many places without rcd protection and it’s classed only as additional protection
Surely seeing a consumer unit without rcd protection should be as serious as seeing a undersized cable or c1
 

darkwood

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When I was a trainee there were no RCDs as we know them in practical use in installations however what may seem counter-intuitive is that fatality from electrocution wasn't very much different to today, the main cause of death from electrical installations and products is usually the fires they cause which is still the case.
 

telectrix

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RCDs are used where ever there is a danger of persons coming into contact with live parts.

first, was sockets for outdoor use..... cutting mower/hedgetrimmer cables leaves exposed live parts, probably on wet grass, operator picks up cut cable.... dead.

second, was more sockets, damaged appliance flex exposing live conductors.operator dead.

third.. cables buried in walls, homeowner drills for new shelves, hits cable..... dead.

and so-on and so-on.
 

static zap

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Advent Win
first, was sockets for outdoor use..... cutting mower/hedgetrimmer cables leaves exposed live parts, probably on wet grass, operator picks up cut cable.... dead.

third.. cables buried in walls, homeowner drills for new shelves, hits cable..... dead.
The lack of easy to find statistics is quite predictable--
No DIY store will sponsor their collection ..
May put punters off buying power tools
.... (spice racks are dangerous)
 
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