|
|
|
|
|||||
|
||||||
From: Arild Jensen (no email)
Date: Mon Jul 01 2002 - 03:01:45 EDT
Norm wrote:
> The second rating is the maximum current the breaker can stop. Above this
> current the arc inside the breaker will continue. This huge current will rarely
> be encountered because it just is not available through the ac wires which limit
> it by their resistance. This is the reason that 12 VDC breakers must have much
> higher max interrupt ratings - they have massive wires so much more amperage is
> available.
REPLY
Norm you forgot to mention soemthing.
DC breakers will have a harder time interrupting a DC current compared to
the same current value in AC.
The reason being that alternating current crosses the zero point sixty or fifty
times a second. This helps extinguish any arc wheras a DC current maintains
a constant arc intill the gap between the contact points or the arc itself cools
off enough to extinguish.
In many DC breakers and relays a magnetic arc blowout is included to
expedite the turn off process.
While Your comemnt about the resistance of the smaller AC wires is partially
true, another factor counteracts this. The shore power utility grid has an
almost infinite capacity to provide current compared to either a battery or a
genset.
When you create a short circuit in your boat's internal supply the voltage will
rapidly collapse as the capacity of the genset inverter or battery is exceeded.
If the same short circuit occurs when you are conencted to shore power, the
grid can continue to provide voltage and current for much longer.
Consequently, shore power breakers should be Class C and be able to
interrupt 7500 or 10,000 amps whereas a normal branch circuit breaker only
needs to be rated as Class A and have a rating of 3000 amps.
Finally, your comment about using breakers poses some risk, if you get the
right combination of cross wired circuits, you may produce such a great fault
currrent that ordinary breakers fail to break the arc quickly enough.
What is normally done in properly designed and built yachts is an interlock
system either mechanical or electronic which prevents dangerous cross
conenctions.
ABYC requires that the neutral as well as the hot line be switched and the
contqact be broken befoer the next contact be made.
This prevents dangerous fault currents from being created and routed
through the neutral wire.
North America, in contrast to other parts of the world allows unswitched
neutrals. As long as the neutral is grounded and sized appropriately, that is
fine buit in a boat you have the possibility of an ungrounded neutral as well as
possible polarity reversals.
Norm's approach may be acceptable if the person who is doing the wiring
understands the intricacies and the operator/user follows a set procedure.
However, this approach is potentially very dangerous for someone lacking
expertise and indepth knowledge of electrical systems.
regards
Arild Jensen
___________________________________________________________________________
|| The Live-Aboard List : send a "subscribe" or "unsubscribe" request ||
|| in body of message to: ||
|