MCB and welder help needed

[beenbeen300]

Hi, I have an Oxford welder (big old one), which I want to wire into the existing consumer unit in the garage ..the unit is a steeple with one 80 amp rcd and two 12 amp mcb's and a number of free bays, what I really want to know is how easy is it to wire a new 40 amp mcb into a free bay ?

Thanks Andy

 

[Steve]

where does the consumer unit get its power? what protective device is at the source?

 

[beenbeen300]

It gets the power from a large spur with a tripswitch and fuse in a nearby plant room.

Andy

 

[Spark123]

It might be possible providing the supply is adequate, as you mention plant room I envisage you are talking commercial/industrial? It may be better for you to get a qualified electrician in to install it. A welder may need a type D breaker which is not to be taken lightly, the efli needs to be very low.

 

[industryspark]

A welder may need a type D breaker which is not to be taken lightly, the efli needs to be very low.

very much agree with that.

seen tons of examples where this has been looked over.

this isnt a diy job in my opinion. do you no about earth fault loop impedance and how to measure/calculate it?

what I really want to know is how easy is it to wire a new 40 amp mcb into a free bay?

easy (if you no what your doing) the trick is knowing what you can/cant do and whats suitable for the job and with the nature of what your wanting to do it sounds like its beyond you.

sorry to put damper on things andy but thats the bottom line.

follow sparks advice and get an electrician in to do the job. better safe than sorry.

 

[beenbeen300]

thanks for the replies,

The plant room is a water treatment plant (we are on a farm and have a borehole), the spur is fused with a 100 amp fuse and this runs the lighting and sockets in the shed.
I did try to get an electrician, but everyone seemed to be very busy, I went ahead and did the job myself..and the welder seems to work fine..is there something eles I should check ?
Now I am worried about efli (You are correct I had never heard of it)
How is it calculated , what do I need to know ? (I will get a sparks to check the wiring as soon as I can, but it did seem fairly starightforward...have Ii missed something ??

Thanks for your patience

Cheers
Andy

 

[toasty]

ELFI is the earth loop impedance. You need special kit to measure it correctly/safely although I have done so in the past using by connecting a large lamp across the live and earth (not to be recommended unless you know what you are doing) and using a multimeter to measure voltage and current.
The implication of the impedance being too high is that the MCB won't trip in a timely fashion (or at all!!) and may result in a dangerous potential appearing during a fault condition.



Did you by any chance get the welder off ebay? If so nice buy!! I almost bid on it myself, but ended up buying a more portable model, but if I had the space I would have gone for the Oxford.

 

[beenbeen300]

thanks toasty,

i bought it locally (devon)..a bloke was moving house and wanted thirty quid ..
It does seem to work okay, the mcb did trip once but all seems well, do you think that that means all is well (I'm worried now)

Cheers

Andy

 

[Spark123]

It's impossible for us to say, an MCB operates on both magnetic (fault current) and thermal (overload current). Even if it operates on overload current doesn't mean it has a sufficiently low earth fault loop impedance for it to operate on fault current. You'd be better having an electrician come and have a look to check it is OK.

 

[RMS]

(I'm worried now)

Thats the risk you take when carrying out this sort of work.

If your unsure then its best to have it checked out by an electrician. You'll feel a whole lot better.

 

[beenbeen300]

I am sure you are right and i will do that,

Even if it operates on overload current doesn't mean it has a sufficiently low earth fault loop impedance for it to operate on fault current

what does this actually mean in operation??


Andy[/quote]

 

[Spark123]

It means even though the mcb trips if your welder goes faulty and starts taking too much juice, if the live wire touches the body of the welder livening it up it might not trip fast enough or at all to remove the danger.

 

[beenbeen300]

thanks spark 123, would it be the case then that a D type breaker would trip sucessfully in those circumstances ?


Andy

 

[Steve]

thanks spark 123, would it be the case then that a D type breaker would trip sucessfully in those circumstances ?


Andy

YES, IF the ELFI is low enough!

 

[Spark123]

A type D mcb does not trip with high startup currents such as those from a welder which is the reason I suggested this is the appropriate type. For a 40A type D MCB the EFLI measured at 20deg C should be no more than 0.24ohms, which can quite easily be lower than that of your supply. Without checking it there is no way of telling.