Surge suppressors

Bob Drzyzgula bob at drzyzgula.org
Sat Nov 2 10:28:44 EST 2002


On Sat, Nov 02, 2002 at 06:32:36AM -0500, Robert G. Brown wrote:
> 
> On Fri, 1 Nov 2002, Bob Drzyzgula wrote:
> 
> > I belive that a "15A" outlet, e.g. a 5-15R, is designed to be able
> > to safely carry at least 20A. Again, it isn't a matter of what
> > will melt down the outlet's internal wiring, it's a matter of 
> > the hole pattern in the front of the outlet.
> 
> That makes sense, and is what I had actually thought/hoped was the case
> (although I didn't know the details about plug format and hole pattern).
> So when they say a 15 amp circuit can have no more than a 15 amp
> receptacle, it just means that you can't put in a plug that could accept
> a 20 amp appliance.  Admirable.  If all "15 amp" receptacles can
> actually pass 20 amps without melting down, even better, although that
> picture I found on the web suggested that this is not strictly true.


I find that hard to believe; I'd be interested in seeing
that picture you speak of.

A receptacle needs to add a "minimal" amount of
reistance to a circuit. The question then becomes
what means "minimal".  One possible way to think of
it is by comparing it to the contact resistance of the
spring-clamped terminals -- I would think that you'd want
the contact resistance to dominate the overall resistance
of the device.

I've tried but can't find the specs for the contact
resistance of typical NEMA receptacles.  However, specs for
IEC connectors -- like the ones on the backs of computer
power supplies with removable line cords -- run around
10 to 30 milliohms (initial), from what I've seen. Some
"high-current" (27A or more) but non-NEMA connectors I
found had a contact resistance of around 7 milliohoms.

So, perhaps we could be conservative and say that, for a
NEMA 5-15R, the contact resistance is maybe 5 milliohms.
To meet the contact resistance dominates wiring resistance
criteria, say that the the internal wiring resistance of
the receptacle needs to be no more than a tenth of that,
or a half a milliohm. Guess also that the length of that
wiring is maybe an inch. On an Ohms per foot basis, this is
about equivalent to an 18AWG (about 1mm diameter).  I don't
know about you, but most 5-15Rs I've seen look to have
conductors significantly fatter than an 18AWG equivalent.

At 15A, BTW, a device with 5 milliohms of resistance would
dissipate 1.125W with a voltage drop of 75 mV. At 20A,
it would dissipate 2W with a voltage drop of 100 mV.
In neither of these cases are you going to melt the
receptacle. In fact, whatever the resistance of the
receptacle, a 20A load will cause it to dissapate about
1.8 times the power that a 15A flow will cause. How much
power dissipation do you think it will take to cause a
meltdown of that 5-15R? Do you think that people would
find more than half of that to be an acceptable normal
state for a 15A outlet?

Certainly the outlet would get unacceptably hot at
100W. For that to happen at 20A, though, the device would
have to have around 250 milliohms of resistance. For
an inch-long copper conductor to have this kind of
resistance, it would have to be no more than about 31
microns thick. The only way I can imagine this happening in
a properly-installed 5-15R would be if somehow the springs
failed and the plug no longer got properly clamped into the
receptacle, raising the contact resistance to dangerous
levels -- i.e., we're talking about a failure mode, not
an engineered design.

--Bob
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