On 22 December 2010 16:07, Sean Breheny <shb7@cornell.edu> wrote:
> My recent experience was with safety contactors so Jonathan is right.
> I do not know whether the contacts-in-series trick can be done with
> other types. However, I think that the mechanism of action here
> includes arcing. In other words, placing several contacts in series
> doesn't eliminate arcing, it merely reduces the maximum duration of
> the arc to the time it takes for all of the contacts to open. When the
> first one opens, an arc will start and there will be a voltage drop
> associated with it. When the second one opens, there may then not be
> sufficient remaining voltage to sustain an arc across BOTH sets of
> contacts and if so, the arc will cease.
>
> Sean
>

One other effect worth noting is the difference between a relay and a
contactor. With a contactor designed for reasonable current, the
mechanical operation is spring loaded with a bit of play deliberately
built in. This way, on disconnect the armature has got a fair amount
of speed up before it impacts the contact carrier, pushing the
contacts apart at a higher rate to minimise any arc.

And in some cases it may be desirable to delay the disconnect.
Fo example in case of a fault condition, it's generally better to make
sure the fuse or breaker has opened before the contactor disconnects.
Otherwise the fault current can create a significant  arc.

One other effect of high fault currents can be that the magnetic field
that develops can tend to force the contacts open. So the coil current
needs to be sufficient to withstand this additional force.

High current DC contactors can also have magnetic blowouts also, which
blow the arc outwards, increase its length and keeping it away from
the metalwork. It only works if the current is only passing in one
direction - not recommended for battery connections.

RP
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