Circuit protection (was: RE: Latest addition : PDP 11/70)

From: Chris Kennedy <chris_at_mainecoon.com>
Date: Wed Aug 1 14:13:12 2001

Tony Duell wrote:

[snip]

> Does this mean you can have the rediculous situation of a piece of 3A
> mains cable (say for a table lamp) being protected by the 20A fuse in the
> fuse box? Ouch!

Certainly in the US. Typical branch circuits are 15 or 20A at a nominal
115V (although voltages between 110 and 130 are common, largely as a
function of how the single phase was derived). The NEC doesn't consider
the fact that a given appliance that's plugged into such a branch circuit
may have an attached cord that is rated at less that the rating of the
branch to be an issue; it simply requires that in the absence of a
overcurrent that the rating of the cord match be appropriate for the
current draw of the appliance. The rational is that since the NEC also
requires fast-acting circuit protection devices with the ability to
interrupt
at least 10,000A that a catastrophic failure won't have time to cause
significant heating in the cord.

Of course this doesn't address the issue of devices which may fail at a
current draw that is substantially above the devices nominal draw but
less than the rating of the branch -- but then neither do individually
fused outlets unless you are careful to change the fuse each time you
connect a device -- which I've certainly never observed happening in my
visits to the UK. Instead almost all appliances and devices are required
to have some level of internal protection -- be it fused, breakered or
thermal -- before they can earn a safety rating (in the US we're typically
talking about UL) -- without which it's very difficult to actually
_sell_ something.


--
Chris Kennedy
chris_at_mainecoon.com
http://www.mainecoon.com
PGP fingerprint: 4E99 10B6 7253 B048 6685  6CBC 55E1 20A3 108D AB97
Received on Wed Aug 01 2001 - 14:13:12 BST

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