Brian, I can only speak about EN 60950, but I think the basics apply to all
equipment. There are two types of faults for which there must be protection:
earth faults and overload.
Each line of the mains must be protected from earth faults. That protection
may be in the equipment, or if allowed by the standard, that protection may
be in the building wiring.
The equipment must include over current protection for each phase. If you
only have one phase, only one protective device is required. Usually that
protective device is a fuse. To prevent unexpected hazards to service
persons, that fuse may not be place in an identified neutral line.
So, the end result for "normal" low power pluggable equipment is that the
equipment has one fuse for over current protection and it is located on the
"hot" side of the line and there is a breaker/fuse in the building mains for
earth fault protection.
Having said that, the UK and some other parts of the world have a power
mains setup that allows for heavy earth fault currents from either pole. So
they use dual fused power cords.
Richard Woods
----------
From: brian_kunde [SMTP:[email protected]]
Sent: Friday, October 06, 2000 12:32 PM
To: emc-pstc
Subject: Overcurrent Protection: One or Both Sides?
The issue of designing in Overcurrent Protection on one side or both
sides of
the AC mains keeps coming up. We deal in Laboratory Equipment so
the EN 61010-1
is the standard we use. I'm not sure what EN60950 would say on the
subject.
The only area I have found that deals with this question is a NOTE
in section
9.6 of EN61010-1 which says, "Overcurrent protection devices (e.g.
fuses) should
preferably be fitted in all supply conductors."
This seems "GRAY" to me and I get beat up on it all the time. I
feel that
overcurrent protection should be on all current carrying conductors.
With a
230V~ product you never know where in the world the product will be
shipped, if
the AC Main has a grounded neutral, or if the receptacle is
polarized. So, I
feel you never know for sure which line or if both lines will be
"HOT" in
reference to Earth ground.
If my thinking is correct, shouldn't ALL 230V~ products have
overcurrent
protection on both sides of the line? I would think so, but I see
products
everyday that only have ONE side of the line fused. My superiors
feel that if
others can get away with it, why can't we. Why add the extra cost
of double
pole breakers or double fuse holders if it is not necessary?
Am I being too cautious or do I have a point?
Thank you for your support and advice.
Brian Kunde
LECO Corp.
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For help, send mail to the list administrators:
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