Magnetic fields

[properjob]

I am in the process of installing a 230v AC ring main in my Prout Catamaran and it occurs to me that I am in effect creating a closed metal ring of about 3 metres diameter. Will this set up a magnetic field inside the ring when AC is flowing?

Would this have any effect upon any magnetic device, for example the fluxgate compass of the Autopilot which is inside the ring or the hand bearing compass that is stowed inside the ring ? I know that 230v AC would not be flowing when I am sailing but if a magnetic field is set up by the AC flowing would it have any residual effect upon any electronic or steel items kept inside this ring ? Could it alter the deviation of the steering compass ?

I should know the answer to these questions but my schoolboy physics is somewhat rusty and I am getting confused. Advice please.

 

[Billjratt]

AC mag fields don't leave a residue, in fact they degauss as they collapse on turn-off, so you'll be cleaning the ship (and theoretically your mag compass needle) but it'll be minimal. I don't think you could measure it. Bear in mind when you draw current from a socket the power running to it will go down both sides of the ring(and oppose). It will never go "round" as such.

 

[catmandoo]

you would also have to be drawing current at the time

 

[wotayottie]

You will set up a magnetic field when the AC is flowing but this will stop when the current stops. You wont leave a magnetic field induced into anything.

Dont forget an RCD

 

[bobgoode]

Any field produced by AC will be varying at the mains frequency. Furthe it will be minimal even for large currents as the live, carrying a current in one direction, is largely cancelled by the field of the neutral, carrying a return current.

Don't worry about it.

 

[VicS]

[ QUOTE ]
AC mag fields don't leave a residue, in fact they degauss as they collapse on turn-off

[/ QUOTE ] Not quite true. If you use a coil to demagnetise something you either slowly withdraw it from the coil or gradually reduce the current in the coil to zero before switching off.

 

[savageseadog]

I managed to corrode two of my winches by putting a turn of shore power cable round them!

 

[William_H]

You Whaaaaaaaat?

I would need a fair bit of explanation of fields and induced currents to accept that.

Re the original poster all of the above is corret. The fields from the currents in the active and neutral will cancel one another being in opposite directions. The current from, the ring main will be shared perhaps by the two sides of the ring. Hence it could not work like a one turn coil.

If the AC did set up a huge field it would tend to reduce the permanent magnetic field of compass etc except that AC is usually turned off at a current part of the cycle hence may finish with a field like a DC field but of any polarity or strength depending on the instant of turn off.

But there will be no field so don't worry. olewill

 

[savageseadog]

It happened OK, no doubt at all, I'm an electronics engineer and I'm not mistaken.
I agree that the nett field should be zero but the proximity of the metal in the winch to the cable means there is a small local field which corroded the winch. There will probably be a small imbalance due to leakage somewhere, a handful of milliamps? but enough to electrolyse metal. One or both effects come into play. AC currents will cause electrolysis, AC is zero volts DC over a full cycle but on one half of the cycle and the other half will not be electrochemically neutral, just to cover that apect of things

 

[William_H]

Well thanks for that. I am not in a position to make any more comment with any certainty so I will store it away as a possible.....
Did it take a long time to corrode the winch? olewill