[Beowulf] Quick question...

Simon Kelley simon at thekelleys.org.uk
Sat Mar 17 13:47:02 EDT 2007


Robert G. Brown wrote:
> As I'm working up a server room and some UPS's, a question arises.  The
> APC units we have have a different plug and current rating from the
> circuits in the wall.  In particular, we have L6-30R receptacles and the
> APC requires only 20A.  I had thought that it would be perfectly safe to
> replace an L5-20P with an L6-30P and plug a 2200 VA UPS into a 30 amp
> circuit, but the electrician who was doing some of the rewiring said "we
> can't do that" because "the 30A circuit breaker might not blow in time
> to protect" the 2200 VA device from damage in the event of an internal
> short.
> 
> This argument seems quite senseless to me.  If this argument were
> correct we could never plug a 15 A device into a 20 A circuit, etc.  It
> would really simplify my life if I can just rehead the cable.  Do any of
> the electricity-savvy people here have any comments regarding the safety
> or legality of doing so?  I have always assumed that going up a size in
> circuit capacity is generally safe and legal...
> 
> I'm planning to do the rewiring of cables today soon, so if you know
> something about his please respond "soon".
> 
>     rgb
> 

Disclaimer: I come from the Right side of the Pond: the electricity here 
is different.

Your electrician is probably right: though possibly for the wrong 
reason. The UPS should have internal protective devices so it should be 
safe. The problem is the cable from the receptacle to the UPS. Consider 
reducto-ad-absurdam and imagine that the cable is really, really weedy, 
like zip cord, for instance. Now imagine a short circuit in the cable 
close to the UPS. The cable is small cross-section, and therefore 
highish resistance, which limits the current flowing. If the cable 
resistance limits the fault current to less than that needed to trip the 
breaker, then breaker won't trip and the cable will get hot and burn 
down your machine room. Paradoxically, a high fault current is safer 
than a low fault current.

It follows that there exists a cable size and length which is safe for a 
20A breaker (because the fault current is >20A) but not safe for a 30A 
breaker (because it's resistance limits the fault current to <30A) That 
cable size happens to be the most economical one that your UPS 
manufacturer can fit. Oops.

So, probably not safe. Legal? I dunno, that's between you and your 
government.

As an aside, this is why 230V electricity over here, though more likely 
to kill you through electrocution, is less likley to burn down your 
house. For a given power, we can use a lower value breaker, and the 
extra voltage increases the fault current, so the breaker is doubly more 
likely to blow when there is a fault.  For really extra special 
specialness, here in the UK we have fuses in the plugs themselves, so we 
can have weedy appliance cables, 32A (at 230V) breakers, and still not 
burn the house down (mostly, as long as the correct size fuse is fitted 
in the plug).

For the gory engineering details, Google "adiabatic equation" or "energy 
let-through".

Cheers,

Simon.

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