Please ensure your electrics are up to date!!!!!!!!!

waynehayes

dwarvenassassin wrote: »

Voltage is not the killer. It is current flow that kills. Not many people realise that only 120mA is FATAL. That's a fraction of an amp.

I'm halfway through an electrical installation college course, and we are taught that only 50mA is enough to stop your heart. Indeed, we are taught 50mA at 50 volts for 50ms can be fatal. Apparently the nazis did extensive testing during the 1940's and no-one survived these figures.

AdrianW2

waynehayes wrote: »

I'm halfway through an electrical installation college course, and we are taught that only 50mA is enough to stop your heart. Indeed, we are taught 50mA at 50 volts for 50ms can be fatal.

Hopefully by the time you've finished the course they'll have taught you why that's a meaningless load of half-truths.

waynehayes

AdrianW2 wrote: »

Hopefully by the time you've finished the course they'll have taught you why that's a meaningless load of half-truths.

Care to elaborate a little? I suppose they teach low figures to encourage caution.

AdrianW2

waynehayes wrote: »

Care to elaborate a little? I suppose they teach low figures to encourage caution.

Not criticising the low figures. A tiny current can be fatal if you're unlucky enough to get it at the wrong time directly across your pacemaker.

The risk of injury depends on many things, notably where the body is shocked and at the frequency of the current as well as energy. At relatively low voltage/currents the big risk is heart rhythm failure because they will be fatal even if the victim lets go. That depends on the frequency of the supply, but you'd be very unlucky to get hit at the right part of the cardiac cycle to die from 50mA for 50ms. 200mA 50Hz for 2 sec is likely to be fatal, 200mA DC not a problem.

If you're working somewhere dangerous, a 6mA shock that causes paralysis may be fatal. Similarly a low 30mA current shock that causes respiratory failure would be fatal if breathing wasn't restored.

To get currents of this order in the first place at 50V you'd need the skin to be soaked in water.

On the other hand, you could survive a 1A shock with only burns if it managed to miss any vital organs - that's why you should arrange your work to avoid the risk of shocks across the chest.

Electricians are the high risk group for electrocution, in as much as there is a high risk. Householders are at much more risk from fire and even an electrician is much more likely to be killed in his/her van on the roads.

I don't claim to be an expert in this at all, this is just my interpretation of the various guidance on the web.

Ugi

This was my thing about voltage - I don't question that it's current that kills, anyone who has received a shock from a nylon carpet can confirm that voltage in itself is not an issue. What you need to avoid, however, is sufficient PD to generate a particular current across your chest. The resistance/impedance of a body is significant and the 100V is, I believe, less than a typical body requires to receive a fatal current, and hence the old boy who was able to "feel" the fuses. This signaling current may also have been DC, which the above poster pointed out is not necessarily so much of a threat to cardiac rhythm.

I wonder out of curiosity why 50 Hz is an issue when 50/100 bpm would be safe for the average heart. I guess it's causing un-coordinated contraction which is the issue, but then why should AC be worse than DC. Just interested, if anyone happens to know about the physiology of these things. Apologies if that's too off-topic.

dwarvenassassin

DC is more dangerous than AC.
The heart is a series of muscles that contract in a specific way. You put a DC current across those muscles and the 'lock up' (sorry, not a very technical explanation) and stop pumping. Therefore AC frequency is not a factor.
If the muscles are damaged by the current, the heart can never restart itself.
BTW, do you realise that Defib machines are not designed to start the heart but to stop it! The hearts built in pace maker then restarts sinus rythm.

If DC was less dangerous then we would not have AC in homes. As many many Electricians and apprentices will agree, 500V DC across the chest wil;l make all the muscles contract noticably. (IR Tester anyone?)

AdrianW2

dwarvenassassin wrote: »

DC is more dangerous than AC.
The heart is a series of muscles that contract in a specific way. You put a DC current across those muscles and the 'lock up' (sorry, not a very technical explanation) and stop pumping. Therefore AC frequency is not a factor.

Sorry, I can't let this pass. The heart requires about 5 times more current to go into VF with DC or 1kHz AC than it does with 50Hz AC. Frequency is very much a factor.

The 50/60Hz mains frequencies came about for practical reasons related to efficient distribution, lighting flicker and motors, not safety.

One of the important things about being a "competent person" is knowing how far your competency stretches and there's a reason why heart surgeons don't qualify after a 5 week course.

Mary_Hartnell

AdrianW2 wrote: »

Sorry, I can't let this pass. The heart requires about 5 times more current to go into VF with DC or 1kHz AC than it does with 50Hz AC. Frequency is very much a factor.

The 50/60Hz mains frequencies came about for practical reasons related to efficient distribution, lighting flicker and motors, not safety.

One of the important things about being a "competent person" is knowing how far your competency stretches and there's a reason why heart surgeons don't qualify after a 5 week course.

Seriously don't we all enjoy Holby/Casualty; they raise someone from "the dead" every week?.

Here is a translation of the VF jargon:

http://www.wellsphere.com/general-medicine-article/the-chain-of-survival-hellip-it-nbsp-works/489107

The little visual of what to do looks like good advice.

urbanleaf

Great thread:T

montag451

I agree its a great thread,

I love the guy that thinks every spark is a scaremongerer that's just trying to rob people blind under the banner of " your house is going to burn down".

I had to look a fault at a house last week

While there I saw the state of their ancient fuse board ,

Rewirables with 1.5mm cables doing the bedroom ring in a 30amp fuse .

Fuse board stuffed full of cables in the "birds nest formation "cover permenantly open unable to shut it because an 'energy monitor' clamped round the main neutral, installed by god knows who.

It was so bad I took a picture to send to the landlord.




Obviously I advised them to get the board changed and sort out the wiring,

will he do it or ask someone to?

Probably not .

You see stuff like this every week in the trade if your doing call outs.

Point is , if you see something as dodgy as this do you walk away and say nothing or tell the customer the truth and look like your trying to create work for yourself.

Is it not an obligation to tell people of the potential dangers in their home ?

It's not just about fleecing people dry.