RCBOs

[stillp]

if a 'leakage' large enough to trip an RCD but not large enough to trip an OPD (fuse, MCB or whatever) results in something touchable becoming 'live'. In that situation an RCD will hopefully clear the fault before anyone has a chance to touch the live object and get a shock

Yes, that would prevent a shock.

Another thing to consider is that the sensitivity to electric shock varies from person to person - ISTR the 30mA threshold was selected because that will prevent fibrillation in 95% of adults under 'normal' conditions.

 

[JohnW2]

Another thing to consider is that the sensitivity to electric shock varies from person to person - ISTR the 30mA threshold was selected because that will prevent fibrillation in 95% of adults under 'normal' conditions.

Yes, something like that.

It would obviously be impossible to pick a threshold that would prevent fatality in everyone. Some people are walking around (often unknowingly) with hearts so electrically unstable that they are forever at risk of developing ventricular fibrillation spontaneously. In such people, just a few microamps of shock current could be enough to represent a 'last straw'.

There obviously has to be a degree of compromise. One could reduce the threshold to 10mA, or even 5mA, in order to increase that 95% (or whatever), but such devices would probably be tripping all the time due to the small leakage currents which are inevitably (and increasingly) created by attached loads.

However, I still have to wonder, even with 30mA devices, how many people have actually experienced and survived a shock which caused an RCD to operate!

At the other extreme, in relation to the mechanism of 'shock prevention' I mentioned (clearing a fault before anyone has a chance to get a shock), a 100mA threshold would probably usually be adequate.

Kind Regards, John.

 

[Simon35]

Until the problem occurs of course. Then you have major appliances out because of say a small fault on a lighting circuit. You sound like someone who castigated RCDs when introduced, when many said, "we had no problems so why introduce them?"

You know what? Most safety and security systems will never be used. But when they are, they make a difference. RCBOs are a form of insurance, as are security video cameras.

An RCD and an RCBO offer the same in terms of tripping on high leakage currents. One is no safer than the other. I can't recall ever having any of the MCB's in my consumer unit trip, and I doubt it's such a regular occurance in other households as to make it a worthwhile spend.

What makes you think I'm not in favour of the use of RCD's in any statement I've made on this thread?

Do you have a full board of RCBO's? How often do test them properly, not just pushing the trip button?

 

[Simon35]

Yes, something like that.

It would obviously be impossible to pick a threshold that would prevent fatality in everyone. Some people are walking around (often unknowingly) with hearts so electrically unstable that they are forever at risk of developing ventricular fibrillation spontaneously. In such people, just a few microamps of shock current could be enough to represent a 'last straw'.

There obviously has to be a degree of compromise. One could reduce the threshold to 10mA, or even 5mA, in order to increase that 95% (or whatever), but such devices would probably be tripping all the time due to the small leakage currents which are inevitably (and increasingly) created by attached loads.

However, I still have to wonder, even with 30mA devices, how many people have actually experienced and survived a shock which caused an RCD to operate!

At the other extreme, in relation to the mechanism of 'shock prevention' I mentioned (clearing a fault before anyone has a chance to get a shock), a 100mA threshold would probably usually be adequate.

Kind Regards, John.

A project I worked on last year had an RCD on the main supply to our plant. Our equipment had a number of variable speed drives for electric motors, which always have high leakage currents and always trip RCD's. We removed the rcd from the supply and put in large earth cables to reduce the touch voltage in the event of a fault such that the current would be less than 5mA in a typical person. Above 5mA it was considered that somebody might drop a tool or whatever in the event of a shock and that could not be tolerated in this particular workplace.

 

[JohnD]

I am not up to date with this, but I happened to see some papers on RCD sensitivity and timing and their effect on safety. The 30ms/30mA spec was considered to permit a shock so short in duration that it would not interfere with the beating of a normal person's heart. Usually the circuit will break in less than one cycle, before the voltage reaches its peak.

 

[JohnW2]

.... We removed the rcd from the supply and put in large earth cables to reduce the touch voltage in the event of a fault such that the current would be less than 5mA in a typical person.

My goodness! ...

I don't know what impedance you were assuming for a 'typical person', but if you were assuming, say, 1kΩ, that would be a maximum touch voltage of 5V. With a 230V supply, and assuming L and CPC paths were the same length, I reckon that means that the CPC would have to have a CSA 45 times greater than the CSA of the L conductor!

Kind Regards, John

 

[JohnW2]

I am not up to date with this, but I happened to see some papers on RCD sensitivity and timing and their effect on safety. The 30ms/30mA spec was considered to permit a shock so short in duration that it would not interfere with the beating of a normal person's heart.

For a start, I'm not sure where your 30msec comes from. Although I agree that they generally do operate much faster than they 'have to', standard RCDs/RCBOs are required to operate in 300msec at IΔn and 40msec at 5 times IΔn - that's 15 voltage cycles possible maximum trip time at IΔn. However, even 300msec is only about a third of a cardiac cycle.


As stillp has said, statements like "...that it would not interfere with the beating of a normal person's heart" have to be phrased in probabilostic terms (e.g. "... the normal beating of 95% of normal persons' hearts"), since there is so much biological variation, even in apparently healthy people.

Usually the circuit will break in less than one cycle, before the voltage reaches its peak.

Even if it did break the circuit in less than 20msec (even though only required to do so in 300msec), it doesn't really work like that, does it? It all depends upon the point of the voltage cycle at the moment the shock starts. If, by chance, the shock starts at/about the time of peak voltage, one will experience that peak voltage straight away. The same 'chance' factors also apply in relation to the cardiac cycle, since the heart is much more sensitive to electrical stimulation at some points in the cycle.

Kind Regards, John

 

[Simon35]

My goodness! ...

I don't know what impedance you were assuming for a 'typical person', but if you were assuming, say, 1kΩ, that would be a maximum touch voltage of 5V. With a 230V supply, and assuming L and CPC paths were the same length, I reckon that means that the CPC would have to have a CSA 45 times greater than the CSA of the L conductor!

Kind Regards, John

I don't remember the exact resistance figure but it was certainly more than a kilo Ohm. The figure was provided by our end client, I'll see if I can find it later.

 

[bernardgreen]

The resistance / impedance of the human body varies depending on where the points of contact are, the areas of the points of contact and the voltage applied. It can be as low as 20 ohms between the pads of a defibrillator. From the sole of one foot to the sole of the other can be less tha 1 KΩ

A defib does not re-start the heart, it stuns it and stops the fibrillation, the heart then ( hopefully ) re-starts by itself.

There is a lot of information here defib info including a comparison between the energy from a defib and the energy from a TAZER

 

[JohnW2]

I don't remember the exact resistance figure but it was certainly more than a kilo Ohm. The figure was provided by our end client, I'll see if I can find it later.

If the CPC CSA was designed on the assumption of a shock path impedance greater than 1kΩ, then there would be an appreciable risk that the shock current would be greater than 5mA in some people/situations - something you said they were attempting to avoid.

As bernard has said, shock path impedances vary considerably. The internal impedance of the body is very low (being essentially salt-ridden water), the great majority of the shock path impedance being in the skin at the two points of contact. That skin resistance varies according to the part of the body concerned (simplistically, how 'thick/horny' the skin is), the voltage and particularly the degree of 'wetness' of the skin (and 'wet because of sweating' is worse than 'wetted with pure water', because of the salt etc. content of the former) and all sorts of other factors - and is, as one might expect, roughly inversely proportional to the areas of contact. Since the 'internal body impedance' is low, the shock path current is much less dependent on path length than one might expect.,

You will find quite a range of figures around, one of the reasons being the difficulty in collecting data relating to the sort of voltages that interest us. One could safely and 'comfortably' measure body impedance in large numbers of situations in a large number of people if one used a very low voltage - but the results would not be very helpful in terms of, say, 230V. It's not uncommon to see average figures of 1kΩ - 2kΩ quoted for hand-to-hand impedance with dry skin (I think the IEC figures are about that), but that can fall to well below 1kΩ with wet skin, particularly if wet due to sweat. At the other extreme, with very dry skin in highly keratinised areas (like soles of feet), the impedance can be very high (50kΩ+).

Whatever, it will be interesting to know what figure your client provided, if you can find out (and even more interesting if you can discover where it came from). Looking at this the other way around, exactly how massive were these CPCs ("large earth cables") which were installed? Interesting, unless I've missed it, I don't think this is an approach to 'protection against electric shock' which the regs address, or even mention.

Kind Regards, John

 

[aptsys]

There is a lot of information here defib info including a comparison between the energy from a defib and the energy from a TAZER

Some of the information in the paper in that link is horribly incorrect.

 

[bernardgreen]

Some of the information in the paper in that link is horribly incorrect.

Which information is incorrect ?

 

[aptsys]

Which information is incorrect ?

Page 4 about Tasers

 

[Simon35]

If the CPC CSA was designed on the assumption of a shock path impedance greater than 1kΩ, then there would be an appreciable risk that the shock current would be greater than 5mA in some people/situations - something you said they were attempting to avoid.

As bernard has said, shock path impedances vary considerably. The internal impedance of the body is very low (being essentially salt-ridden water), the great majority of the shock path impedance being in the skin at the two points of contact. That skin resistance varies according to the part of the body concerned (simplistically, how 'thick/horny' the skin is), the voltage and particularly the degree of 'wetness' of the skin (and 'wet because of sweating' is worse than 'wetted with pure water', because of the salt etc. content of the former) and all sorts of other factors - and is, as one might expect, roughly inversely proportional to the areas of contact. Since the 'internal body impedance' is low, the shock path current is much less dependent on path length than one might expect.,

You will find quite a range of figures around, one of the reasons being the difficulty in collecting data relating to the sort of voltages that interest us. One could safely and 'comfortably' measure body impedance in large numbers of situations in a large number of people if one used a very low voltage - but the results would not be very helpful in terms of, say, 230V. It's not uncommon to see average figures of 1kΩ - 2kΩ quoted for hand-to-hand impedance with dry skin (I think the IEC figures are about that), but that can fall to well below 1kΩ with wet skin, particularly if wet due to sweat. At the other extreme, with very dry skin in highly keratinised areas (like soles of feet), the impedance can be very high (50kΩ+).

Whatever, it will be interesting to know what figure your client provided, if you can find out (and even more interesting if you can discover where it came from). Looking at this the other way around, exactly how massive were these CPCs ("large earth cables") which were installed? Interesting, unless I've missed it, I don't think this is an approach to 'protection against electric shock' which the regs address, or even mention.

Kind Regards, John

IEC/TS 60479-1:2005 is the standard describing the effects of AC current, in terms of magnitude and time. Shown here

http://www.electrical-installation.org/enwiki/Electric_shock

We had to be within zone AC-2, and less than 50V touch voltage. We arrived at a touch voltage of 40V. The same standard gives a hand to hand impedance of 2600 Ohm at that voltage, and 6500 Ohm hand to foot impedance. That gives approx 6.25mA, and in our case, the OCPD operated in 100mS.

It should be said that this is a very specialized work environment, in Europe, but not in this country.

 

[bernardgreen]

Page 4 about Tasers

I agree, possibly it refers to ( early ) USA TASERs which seem to have been ( maybe still are ) very crude devices with a high voltage source feeding continuously into the target. Modern UK TASERs have a signficantly higher voltage but feed a series of separate pulses into the target so the total energy injected into the target is lower than the USA but with the higher voltage more painful and more disabling.