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? If they are required does anyone know where to get a deep one for a Volex CU?
On Mon, 8 Jun 2009 10:52:34 +0100, "dennis@home" had this to say:
I think that Volex used to make a deep cover, but I've never seen nor used one. As long as you're not likely to bump into the trips you shouldn't have any problems. My last setup was like that.
the cover is normally left off. Be aware that there are downsides to fitting these 3kA MCBs as well as the easy reset advantage.
NT
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!
How do you find the supply impedance without resorting to measuring the short circuit current?
dennis@home coughed up some electrons that declared:
Measurement is the only way.
If you ask your supplier, they'll just say 16kA which is the maximum.
How close are you to your nearest transformer?
Mine's a fair way away and my PFC is about 2kA (measured by me).
OTOH, I had more than 5kA at a flat that has a transformer outside.
Cheers
Tim
Tim S coughed up some electrons that declared:
Brain fart - it's 1.2kA
For the Wylex CUs, you could get covers with openings for the breakers.
The original ones with a pair of circular push buttons were even less, IIRC.
You should use a calibrated tester.
You can probably get a good idea for a max value on a single phase supply by measuring the voltage drop at the CU* when you switch on a known load, such as a 2kW heater or kettle. (That's no good for earth fault loop impedance measurement for checking disconnect times though.) The voltage drop divided by the load current will be the supply impedance.
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.
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)
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