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Topic Title: Main Incomer Fuse Blown
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Created On: 01 February 2013 10:37 AM
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 01 February 2013 10:37 AM
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Docara

Posts: 2
Joined: 30 December 2012

Hi all,

I have just carried out a EIC on a 3ph commercial premises.

The 3ph DB I was working on was protected by a 100A MCCB and the installation as a whole was protected by 200A J type BS88's

On carry out an end to end the earth sprang around and managed to stick itself between a main switch cover and the bus bars underneath feeding the board.

After my shaking hands and heart rate had returned to normal I went to reset the MCCB but it hadn't tripped - what HAD gone was 2 incoming fuses moreover the end of the 2.5 was completely intact.

Now to my question - the MCCB is installed the wrong way round when compared to the others would this mean that it wouldn't trip or is this just a coincidence.

I dont have that much experience on the bigger MCCB's and I just can't work out why/how the main 200A fuse could blew but doesn't melt a 2.5 single or take out the main circuit protector.

Are the 'J' types a fast blow type of fuse ????
Is it worth replacing the MCCB assuming its faulty and installing it correctly???

I would ery much appreciate some opinions

thanks
Matt
 01 February 2013 11:03 AM
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Parsley

Posts: 1032
Joined: 04 November 2004

What have you learnt from this experience?

Was the MCCB adjustable and fitted in-accordance with manufacturers instructions?

Regards
 01 February 2013 11:05 AM
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broadgage

Posts: 1308
Joined: 07 August 2007

The MCCB is probably fine.
They are not normally sensitive as to which terminals are input and which are output.
With an AC supply how would it "know" in which direction the power is flowing ?

There is nothing remarkable in a larger fuse blowing before a smaller MCCB trips. Fuses tend to react quicker to large fault currents such as would be caused by a short circuit near the intake.

At large fault currents conductor heating in the event of a fault is often less than at lower fault currents, since a fuse will clear the higher fault current more quickly.
Had the fault current been only a few hundred amps then the bit of 2.5mm would have got hot and possibly melted.
 01 February 2013 11:32 AM
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Avatar for OMS.
OMS

Posts: 19746
Joined: 23 March 2004

Classic case of partial or no discrimination between fuses and MCCB's. A 100A MCCB operating on fixed thermal/magnetic will almost never grade with a 200A BS 88 fuse - the fuse pre arcing I2t is often less than the current required to achieve definite minimum time

Fuses are effectively constant current for time devices - as you increase the fault current, the faster they operate.

MCCB's on the other hand are mechanical devices - for the same fault current, they need a fixed minimum time to start moving - if the fuse has rached pre arcing limit before this definite minimum time of the breakker is reached then the fusde is going to fully open - the breaker however may not move any further.

For a high fault current and fuses, disconnection can be so quick as to leave minimal damage to even comparatively small conductors -as you've just found out.

The lesson however is "be more careful with loose conductors and live equipment" - it's not a good mix, particularly if you don't have an effective range of PPE on.

I'll just mention EAWR and Reg 14.

14. No person shall be engaged in any work activity on or so near any live conductor (other than one suitably covered with insulating material so as to prevent danger) that danger may arise unless -

(a)it is unreasonable in all the circumstances for it to be dead; and .
(b)it is reasonable in all the circumstances for him to be at work on or near it while it is live; and .
(c)suitable precautions (including where necessary the provision of suitable protective equipment) are taken to prevent injury.


Stay safe, yes

Regards

OMS

-------------------------
Failure is always an option
 01 February 2013 11:42 AM
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Avatar for potential.
potential

Posts: 1276
Joined: 01 February 2007

Yes, work safe!

This potentially deadly mistake provides a good example of fuses operating at there best.
So often I read comment that fuses are not as good as MCBs etc.
You may owe your life to them in this case.
 01 February 2013 11:48 AM
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Docara

Posts: 2
Joined: 30 December 2012

Thank you all for the responses.

Its scary how much you forget after 20yrs+ (when technology starts to come) in and how lucky I was - very humbling.

When you have the client screaming you because of the inconvienience your boss screaming at you for how long it takes, having to leave power on to the remaining 80% of the installation - in the cold light of day doesn't paint things in good light!!!

Genuinly, thank you for the advice and the kick up the back passage
 01 February 2013 12:22 PM
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OMS

Posts: 19746
Joined: 23 March 2004

When you have the client screaming you because of the inconvienience your boss screaming at you for how long it takes, having to leave power on to the remaining 80% of the installation - in the cold light of day doesn't paint things in good light!!!


For sure - we've all been there - on all sides of the fence.

No need to make a drama of it, but take a copy of the reg from EAWR I quoted - just remind clients that they are duty holders under EAWR, and tell your boss he's a **** - if you get zapped, his actions will also drop both him and the client in the doo dah, the duty is absolute - he'd do well to remember that.

Next time someone says you can't turn off, just ask what they do in a power cut then - if there are a lot of guilty looks, then clearly the installation isn't that critical.

regards

OMS

-------------------------
Failure is always an option
 01 February 2013 04:19 PM
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sparkingchip

Posts: 6171
Joined: 18 January 2003

Luckier than the guy in Marks and Spencers a bit back.

Andy
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