Induced current

[Robotstar5]

I've never thought about it before, but was looking at a car repair near some SRS (airbag) wiring and wondered if the weld arc could induce current into the adjacent wire?.

 

[madkayaker]

Shouldn't if the ground is well made however I would disconnect the wiring just in case as airbags are dangerous if they go off

 

[brightspark]

wrap a bit of lead round the cables

 

[MonsterLandrover]

And earth it! Lol. There is a tiny possibility of that happening but as a mig is DC and very low potential (voltage) the chance of inducing a current even is next to nothing.

 

[Robotstar5]

Shouldn't if the ground is well made however I would disconnect the wiring just in case as airbags are dangerous if they go off

Yes a good ground will help but there is a large amount of current flowing through the arc between the electrode/tip etc and ground so there must be a magnetic field generated?

 

[madkayaker]

Not sure they would be that much as the current is DC

 

[rtbcomp]

It will if the current in the rod/wire varies (AC or DC). This will cause a varying magnetic field which will induce a voltage in nearby cables, think primary and secondary of a transformer.

The amount of magnetic flux depends on current, not voltage and as you're welding with large currents the magnetic field could be significant.

Shielding and earthing will not inhibit the magnetic field, you'll need a ferrous or mu-metal screen.

You will only get a current flowing in nearby cables if they form a closed circuit.

 

[Robotstar5]

I think that answers it, thanks

 

[rtbcomp]

You'll get the greatest effect when striking and extinguishing the arc, the physical relationship of the cables also has en effect.

You get most coupling between parallel cables, least between perpendicular ones.

 

[AndyStobbs]

Airbag wires run in twisted pairs alot of the time, and even when they don't they are always next to each other. Meaning you will induce the same current in each of the airbag wires, but not in the right direction to permit a current flow through the airbag. They need a few amps to set them off.

Disconnect by all means but remember to re-connect before you turn the ignition on, and technically you should remove battery and allow 10mins for capacitors to drain down before airbag disconnection.
All too often now a small mechanical job on a car gives rise to a long winded electrical procedure.
Don't forget about radio codes either, if you do start taking batteries off cars.

 

[normspanners]

All too often now a small mechanical job on a car gives rise to a long winded electrical procedure

Yeh and the customers never 'see'/understand that, that is part of the job and as you say it can take a LOT longer than the job, for instance I did a service on a jetta a few months ago everything went fine 2hrs, last job resetting service warning, connected up the KTS reader reset the 'light' etc 2mins job, no faults recorded etc, switch car off n restart to check, But now the man light on!!! no faults no reason just on. traveled to a mates to try his launch, snap-on, on it just to try to see if they would pick up anything. spent then 3 hrs trying everything checking connections disconnecting battery then losing codes etc etc etc customer says he will have to take it as is, ok I says, he goes out to the car, starts it and said 'what light'? nothing on perfect !!! now I have spent 5hrs total but 3hrs for nothing
PS car did it once more a week latter after filling with fuel and has been perfect since!

 

[AndyStobbs]

Sounds like a horror job. Done/had some myself.
Out of interest how do you find the kts - which one have you got?
I've been looking at the 540 myself, currently use a texa as my day to day tool, which I'm generally very happy with; between that and the picoscope I don't normally want for much else in code reader terms.