uhf ariel

[Jaybird]

I was working on the nav today and had the grill out and also had the uhf ariel bracket lossened off . When refitting the grill had to get the wife to help and during this her bracelet touched the ariel bracket and arched out the burn was quite severe as it stuck to her wrist.
Can anyone tell me why this happened there was no radio on and no ignition key on

 

[baylock]

Static? (tho not likely, it is possible)
Induction? (How close were you working to power-lines, junction boxes, ect, also, how many cores in your antenna, the more wire the more chance)
Check your wiring, see if it runs by the battery, or any other 'live' wiring??

 

[TigerST]

Wow that would have been a rude shock

 

[happy john]

There is no way the radio caused this it simply has not got that sort of power i would guess somehow she has made contact with the positive and the bracelet has arced out on the earth that is the only thing with enough oomph to do this hope she is ok.

 

[Jaybird]

Thanks for replies
she was on the opposite side of car to battery, inside shed holding onto plastic grill and leaning across aloy bullbar when her bracelet brushed against ariel bracket. The charge was enough to melt a pin hole in the bracelet then jump across to a metal watch band.I though it was 240 but checked and there wasn't any near the car.

 

[shan_b]

Is she related to this bloke?

 

[mrtim123]

i'd be double checking your aerial wiring, to make sure it is not the cause of the short. it may be shorting out somewhere else and the result is your wife completing the circuit.

 

[Hugh9359]

Thanks for replies
she was on the opposite side of car to battery, inside shed holding onto plastic grill and leaning across aloy bullbar when her bracelet brushed against ariel bracket. The charge was enough to melt a pin hole in the bracelet then jump across to a metal watch band.I though it was 240 but checked and there wasn't any near the car.

How is the radio wired?Straight to battery or through ign,was ign on?Also static can build up to high levels.Had a car once that shocked everybody that got out of it,fixed with earth strap.Walking around super market pushing trolley with thongs & 1 of my micro fleece singlets on I build enough static that when I reach for something off shelf I arc out,it looks and feels like touching a spark plug with the arc sometimes jumping 40mm,needless to say I don't wear this combo if going to market.Hugh

 

[Jaybird]

The radio is wired through the ignition. This was more than static it was high tension spark and now a couple of days later the burn is the width of her watch strap including the round shape of the back. I have been all over the car in the area it happened using a piece of wire trying to recreate it but nothing have even played around with a multimetre. Weird it is.!!!

 

[Old.Tony]

You don't say what kind of engine you have, petrol/diesel.

First, a little about electricity and "arcing". There are two components to electrical power that we're familiar with - volts and amps. Volts represents the energy level, which is directly related to how far you can bridge a gap with that power. Petrol engines use coils to increase the (rather measly, but very safe) 12V to something in the order of 18,000 volts in order to make a spark jump across a couple of millimetres. A voltage like that can probably jump 15mm to 20mm in the right conditions (humidity helps).

The second component is current or amps, which represents the amount of energy flowing. This is where the true power lies. Welders don't use high voltage - they use low voltage but high current. Current is directly related to how many electrons are being pushed through the wire. The more current, the more electrons and the more electrons, the more heat in the wire - this is how fuses and electric heaters work. Heat generally comes from amps.

If diesel: the UHF aerial cable may have chafed on the positive battery line (either the battery itself, or the cable running to the starter motor). There simply isn't anything else in a diesel's engine bay that can produce enough power, and the contact must be direct since there's no high voltage present that can jump any sort of gap.

If petrol: you can generate a decent spark across a decent gap, but the total power is usually quite low. If I put in 12V at 2A to the coil, and ramp it up (assuming no losses, which is impossible but we'll do it for simplicity's sake) then the total watts in (which must equal the total watts out) is 24W and the output at 18,000V is only 0.0013 amps, which itself can't do any harm. The 18,000V can disrupt nerves in the body and even shock the heart into arrest, but the total power in a car's ignition coil is USUALLY very small.

Unless something is wrong of course - and a chafed connection between the UHF aerial cable and a HT lead could induce power. Enough to cause a significant burn after jumping a gap? That's a LOT of power, it would be a large current discharge through the 12V part of the system and I'd expect a fuse to let go - does your radio work (including sending/receiving clearly)? If it's a petrol car, I'd check the route of the cable, and check the condition of the HT leads.

Otherwise it's a bit of a mystery, unless that radio has something drastically wrong with it.

 

[Jaybird]

Thanks for the explanation old.tony
The nav is diesel, the radio seems to be working OK although havent been able to give it a good try. I've had a good look around and cant see anything obvious but will look further on the weekend as it now has me curious. I, ll post if I find anything.