dennis@home wrote:


"John Rumm" wrote in message
o.uk...
dennis@home wrote:

I want to fit some GE minitrips to a CU that has wired fuses in it.
The trips just plug into the existing slots.
However they are too deep to fit the cover.
Is the cover a requirement or even necessary once the minitrips are
in place?

The lack of a cover is unlikely to be a problem as long as they are
unlikely to be physically abused in their present location.

These plug in MCB often only have a maximum breaking capacity of 3kA
(less than the 6kA or better typical of modern MCBs), but one could
argue that is a non issue since BS3036 semi-enclosed fuses often have
an even lower breaking capacity. Since you are making the change it
would be worth checking your supply impedance to make sure that 3kA is
adequate (if its not, then your existing fuses would almost certainly
be inadequate as well).

Never mind fuses these replace re-wireable fuse holders!

Yup - "semi-enclosed" is the posh name for a rewireable fuse (or
BS3036). The traditional fuse carrier with coloured dots on the back
produced by the shed load by wylex etc.

How do you find the supply impedance without resorting to measuring the
short circuit current?

Most modern test meters will read it directly. However you can make a
decent stab at it with a clamp meter and a DVM.

If you have clamp meter stick it round a meter tail, and put a voltmeter
somewhere where you can see the mains voltage. Then you can measure the
voltage drop against current drawn (although as Andy said - draw the
power from a different circuit to the one you are measuring the voltage
on). Once you have a couple of readings you can just use V/I=R to get
the supply impedance. Anything less than about 0.25 ohms is starting to
get boarderline for a rewireable fuse. A 3kA MCB ought to be ok down to
0.08 ohms - which you are unlikely to meet unless you are in a densely
populated area or sat right on top of a substation.



(if you have not got access to a clamp meter, then you can still get a
feel for it with just a volt meter and a known load - say a 3kW kettle)



--
Cheers,

John.

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