Can you get a shock with a 30mA RCD in place

[happyhero]

Hi, ok maybe silly question but I always wondered.

Assuming everything has been done correctly and you have a newly wired property to the 17th edition, what will happen if you touch a live cable?

Will you feel anything or will the 30mA RCD trip before you can feel anything.

 

[JohnW2]

Hi, ok maybe silly question but I always wondered.
Assuming everything has been done correctly and you have a newly wired property to the 17th edition, what will happen if you touch a live cable?
Will you feel anything or will the 30mA RCD trip before you can feel anything.

Don't try it! You'll certainly 'feel something' (probably a lot) - the RCD can't trip until at least 30mA flows through your body, but that doesn't mean it can't be more than 30mA for a very brief time. By limiting the duration of the shock to a very short period of time, a 30 mA RCD should (but is not guaranteed to) spare you from a fatal outcome if you (and your heart) are in good health - but those with 'dicky tickers' (even if they were unaware of it) could end up in a box!

Kind Regards, John.

 

[echoes]

Depends very much on what else you're in contact with. The spectrum of experience will range from nothing at one end to death by electrocution and/or burns at the other. Somewhere in the middle, the RCD might operate. If you're lucky. Don't try it. Ever.

 

[happyhero]

Cheers for the rapid answers but just to go a little further, what then is the main intension of the 30mA RCD is it to limit shock or protect equipment?

 

[holmslaw]

..

 

[echoes]

A 30mA RCD will disconnect the supply if more than 30 mA flows through one's body to earth as John said, thereby saving one from a larger, more dangerous current.

An RCD offers no protection against a shock obtained across the live and neutral conductors.

 

[riveralt]

Cheers for the rapid answers but just to go a little further, what then is the main intension of the 30mA RCD is it to limit shock or protect equipment?

RCD's are classed as additional protection to minimise your exposure to electric shock. However, as other have said, you would be very foolish to work on electrical installations live and assume that the RCD would save you. 30mA can still kill you if your heart and nervous system are susceptible. It will also depend on where you receive the shock.

Are you planning something?

 

[holmslaw]

..

 

[JohnW2]

RCD's are classed as additional protection to minimise your exposure to electric shock. However, as other have said, you would be very foolish to work on electrical installations live and assume that the RCD would save you. 30mA can still kill you if your heart and nervous system are susceptible. It will also depend on where you receive the shock.

Indeed, and the mode of that 'minimisation of exposure to electric shock'is really the limitation of duration.

There is a common misconception (hopefully not amongst electricians!) that a 30mA RCD will limit the current that can flow through someone's body to earth to a maximum of 30mA. That is clearly not true unless we are talking about a slowly increasing curernt (which we're usually not). As we know, the magnitude of the current is dependent only on the impedance of the path to earth through the victim, and the victim will normally experience that full current (which, in some circumstances, could be a lot more than 30mA) 'immediately'. What the RCD does is limits the duration of any current greater than 30mA to a very short period of time - and that's what, in many cases, may prevent the shock being fatal.

The other very common misconception, which has already been referred to in this thread, is that an RCD can 'help' if one touches both L and N simultaneously - which it clearly cannot.

Kind Regards, John

 

[merlin50]

i have done it, and yes you get one hell of a shock

 

[Alarm]

Also you can have a minor "shock" and drop down dead a week later.

Moral here is check, test, check again, then have another check. And fit lock outs when working away from CU`s.
Had one turned on once years ago when working on a circuit, and the culprit was another sparks FFS. He`s really lucky he is a long standing friend, even so we had severe words.

 

[Paul_C]

The other very common misconception, which has already been referred to in this thread, is that an RCD can 'help' if one touches both L and N simultaneously - which it clearly cannot.

I feel that over recent years there has been a lot of promotion of RCD's which is responsible for this sort of belief. The advertising may not say so in so many words, but I think much of it has been written in a way which tends to leave the layman with the impression that with an RCD fitted it's impossible to be electrocuted or suffer a severe shock.

 

[JohnW2]

I feel that over recent years there has been a lot of promotion of RCD's which is responsible for this sort of belief. The advertising may not say so in so many words, but I think much of it has been written in a way which tends to leave the layman with the impression that with an RCD fitted it's impossible to be electrocuted or suffer a severe shock.

Indeed, and it's clear that many of the public have thereby acquired that belief that they are 'totally safe' with an RCD in place - which obviously caries the risk of complacency and risk-taking. 'Strong promotion' of RCDs is clearly a good thing, but I certainly agree that there should be clear indication of the limitations.

As for the other misconception, I have to say that I've come across a good few electricians (and others who should know better) who haven't thought carefully enough and believe that a 30mA RCD limits the current that can flow through a victim to 30mA.

Kind Regards, John.

 

[sparkticus]

Interesting question and responses. I tend to agree that the RCD appears to have gained a sort of false cult status protecting against fatal electric shock no matter what else is wrong with an electrical installation. A catch all.

I suspect that all those with experience recognise that it only forms part of a protective system (an automatic disconnection system) along with main protective conductors, equipotential bonding (where required) and fully compliant wiring. We have the infamous 50V/1667 ohms =30mA offering hollow assurance that our pipes and other extraneous metals won't kill us.

The 17th edition seems to essentially promote RCD protection on all circuits in the domestic world (you don't see much MIC or 50mm deep cables in an average dwelling these days) which does nothing to undermine the dubious status of RCDs.

My understanding

- A 30mA RCD will do little to prevent electric shock (fatal or otherwise) when a person comes into direct contact with a live conductor. True that the RCD will see the differential current but a person may perceive an electric shock at currents under 1mA and automatically pull away often smashing their hand into something else, falling off a ladder or throwing a screwdriver across the room. Occasionally their heart may go into VF as a result of the current flow across it or simply being startled by the incident. An electric shock drawing currents above or below 30mA may alter the heart rhythm causing problems now or later. Peripheral nerve damage can also be caused by relatively small currents though not common.

- A fatal electric shock may occur well below 30mA. There are many variables and no guarantees.
I have been told about a lab medical study which shows that a current exceeding 50mA as measured across the longitudinal axis of the heart can induce VF but I suspect that theory bares little resemblance to the real world.

- An RCD is intended to automatically disconnect the supply during an earth fault. It is to prevent extraneous conductive parts (pipes, washing machine cabinets, the handle of your electric kettle) from sitting at a potentially hazardous voltage for a protracted period of time.

- A properly installed electrical installation should offer the lowest possible earth fault impedance offering an RCD the optimum opportunity of disconnecting rapidly (not to mention the MCB but obviously at higher currents)

- A servicable RCD will disconnect well within 200ms (often within 20ms) when an earth fault develops in an installation that has been designed with appropriate protective conductors in place and has a reasonably low ELI.

- 20ms is still one whole cycle so if you are hanging on two pipes that are not equipotential then you might see the light sooner rather than later.


- Sometimes an RCD fails to disconnect! have seen a number of failed RCDs. It is not the first line of defence.



The only way I test an RCD is with my multifunctional though I do remember doing it with a resister and scope - those were the days. Having said that, we carry an electric kettle in the van that is so leaky it could be used to test RCDs

 

[Paul_C]

The 17th edition seems to essentially promote RCD protection on all circuits in the domestic world (you don't see much MIC or 50mm deep cables in an average dwelling these days) which does nothing to undermine the dubious status of RCDs.

Nor does the 17th edition's relaxation of supplementary bonding requirements so long as RCD protection is provided. I feel certain that is also reinforcing that "cult status" of the RCD:

"Hey, throw an RCD on the circuit, then nothing else matters because nobody can get hurt."