Touching pos & neg of battery

In the 80s, I was working on a Land Rover which was fitted with a mortar-locating radar system (worked for EMI), and my metal watch strap touched both terminals (24v system, and touched opposing terminals on adjacent batteries). My watch got welded to my wrist and I had 2nd degree burns which saw me off work for over a month, and I still have the scars today.

Don't under-estimate the effect of DC battery amperage.
 
davidej said:
I used to puzzle why the professional advice is to disconnect negative first and reconnect last. It seemed to be counterintuitive.

However i found out why when disconnecting the positive , my metal watch bracelet brushed against some metalwork which was earthed. No serious harm done but a lesson learnt!

Apart from now following that rule, I always take my watch off before starting work on electrics
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+2 Another shorted wriststrap to report!! Trying to by pass solenoid on my Landrover in the middle of a Libyan desert. Skin on wrist pulled off as I ripped the strap off. No A&E. You ARE WARNED !!
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As children do, we used to lick the 9v batteries that cycle headlamps used, it gave a sharp tingling, i wonder what would happen if you connected two wires to a 12v battery and licked that, its only 3v more after all, could someone try it and post the results please.
 
A broken terminal on a 12V battery can be rebuilt using an arc from the battery itself. Roughly: create a metal former round the damaged terminal (use, for example, an old tin can); connect a starter lead to the other terminal; hold the carbon rod from an old dry cell firmly in the crocodile clip; with a solder rod at the ready, use the carbon rod to strike an arc to the damaged terminal; use the arc to melt the solder into the metal former.

Having seen that done, I'm rather less keen on taking risks with such batteries.
 
Thank you to all who replied and apologies for the question being unclear. Anyway, I have an answer. The difference between the human and the spanner is the degree of resistivity, but when the human touches the spanner which touches both terminals, it is a different matter altogether!
 
davidej said:
I used to puzzle why the professional advice is to disconnect negative first and reconnect last. It seemed to be counterintuitive.
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Actually you should disconnect the earth first, which normally means -ve but means +ve in old cars. If you disconnect the live terminal first you can get a short from it to any earthed thing, which in a metal car is almost everything. If you disconnect earth first the only short circuit route is from the earth terminal to any unswitched live.
 
Going back to the 'grab' effect of DC and AC, Im sure the medics here will correct me if Im wrong, but I was taught that the current causes your muscles to contract so that you can not let go. So unless your muscles are able to contract and expand at 50hz and you can move fast enough to break the contact, you are stuck to AC almost as effectively as DC.
 
John the kiwi said:
I am astonished that you can feel 12 volts even with wet hands. You didn't touch an ignition circuit by chance where the voltage is many thousands?
To the OP:
Its the volts that jolt but the amps that kill. ............................
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Like so many of these handy quotes it's a half truth. The voltage and the current are related with a close approximation to Ohm's law.

As a rough guide we begin to feel currents at around 1mA. So whatever voltage is required to force 1mA through a body is all that is needed for a tickle.

TENS devices (transcutaneous electrical nerve stimulation used for pain, nausea relief and muscle stimulation) typically work by discharging a capacitor into the the body. An EN directive defines the maximum voltage and charge which I can't now remember. But the device I designed a few years ago charged a 1uF capacitor to a maximum of 80V. In practice, charging to 5V produces a noticeable effect where the body contact is good. If the contact is less good it's necessary to increase the voltage. However, the full 80V discharge is very noticeable and likely to hurt.:eek:
 
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oldharry said:
Going back to the 'grab' effect of DC and AC, Im sure the medics here will correct me if Im wrong, but I was taught that the current causes your muscles to contract so that you can not let go. So unless your muscles are able to contract and expand at 50hz and you can move fast enough to break the contact, you are stuck to AC almost as effectively as DC.
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None of that has any bearing on what happens with a 12v battery.
 
oldharry said:
Going back to the 'grab' effect of DC and AC, Im sure the medics here will correct me if Im wrong, but I was taught that the current causes your muscles to contract so that you can not let go. So unless your muscles are able to contract and expand at 50hz and you can move fast enough to break the contact, you are stuck to AC almost as effectively as DC.
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Stage lighting technicians are trained to touch the rig with the back of their hand first, so it the rig is live they can pull their hand away. If they grab the rig with their hands, they get locked onto it.
 
oldharry said:
So unless your muscles are able to contract and expand at 50hz and you can move fast enough to break the contact, you are stuck to AC almost as effectively as DC.
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They're not able to expand and contract at that rate. Instead they undergo tetanic contraction, about which Google will no doubt tell you lots. It's what I meant when I wrote of a "ratchet" effect earlier. As also mentioned, the amp threshold for DC grab effect is substantially higher than AC, plus there is no tetanic effect. This also makes a considerable difference on shock effect on the heart (not that 12V batteries carry much cardiac risk.)
 
JumbleDuck said:
Actually you should disconnect the earth first, which normally means -ve but means +ve in old cars. If you disconnect the live terminal first you can get a short from it to any earthed thing, which in a metal car is almost everything. If you disconnect earth first the only short circuit route is from the earth terminal to any unswitched live.
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If the tool you are using to undo the terminal touches pretty much anything metal you have a short if you do the positive first, which is why you should always take the negative off first (negative earth vehicles). If you were using a rubber spanner it won't make much difference either way :)

I've also read on here (no doubt quoted some from ill-informed Goggle search) that the reason for disconnecting the negative first is to get you away from the battery in case of an explosion if removing the clamp causes a spark. This is, of course, a load of nonsense. It's rare for the negative cable to be far away from the battery, even rarer for there to be any explosive gasses to ignite, there has to be a serious fault for that to happen.
 
It is possible to get a shock from a 12v battery, but unusual, especially for me, i'm not as susceptible to shocks as some people. I cannot ever recall getting one anywhere other than on a boat, so the salt water theory may have a bearing. Although i did get a couple of 24v shocks this Summer when i was working in a dry area of the boat, but it was ridiculously hot and i was sweating more water than was in the bilges :cool:

I find it's very rare and hardly life threatening. The worst shocks i've had have been HT shocks, i'd rather have a jolt from 240v than an HT lead, old mag' ignitions are particularly fun :ambivalence:

As for the "Its the volts that jolt but the amps that kill." comment, i agree with John in post #30, it isn't entirely correct. Another good example of when it's badly wrong is with swimmers in the water, very low currents can kill by drowning when the swimmers muscles contract.
 
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Davy_S said:
As children do, we used to lick the 9v batteries that cycle headlamps used, it gave a sharp tingling, i wonder what would happen if you connected two wires to a 12v battery and licked that, its only 3v more after all, could someone try it and post the results please.
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I tried licking a 22.5V flashgun battery when I was a child. Once.
 
Bobc said:
Stage lighting technicians are trained to touch the rig with the back of their hand first, so it the rig is live they can pull their hand away. If they grab the rig with their hands, they get locked onto it.
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I started doing stage lighting as a teenager, and was advised always to keep one hand in my pocket when working on things.
 
By the way, just before anyone brings it up can I mention that the reason you don't get shocks from RF is nothing to do with the skin effect, and all to do with the inability of the body to detect these frequencies because the Na+ and K+ ions can't get from one cell to the next in the time available. The current still goes through you, hence RF burns which tend to be very nasty because you don't feel them happening.
 
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