16 amp help wanted!!

[jonny boy]

After the recent sale of a Jet supersaw, I had both the space and spare cash burning a hole so when the opportunity of a Delta Unisaw for a good price arose, I couldn't refuse it but the problem is I can run the saw with a 13 amp plug on it but the fuse blows after a couple of minutes. I know the remedy is a fixed 16 amp supply but having connected this, the breaker keeps tripping every time it's started, and even the same with a 20 amp breaker. What am I doing wrong?

cheers,
jonathan.

 

[9fingers]

You need to use a class c breaker. These are designed for motor loads.

Same size as standard breakers and usually about the same price. make sure you get the same brabd as the one you are replacing to make sure it fits into your CU.

example:-http://www.screwfix.com/app/sfd/cat/pro.jsp?id=87764&ts=71283

HTH

Bob

 

[jonny boy]

Thanks for your help Bob, now I'll pop into the garage and have a look what class the breaker that's tripping is. Back in two mins.

jonathan.

 

[jonny boy]

It just say's NB116 then a 16 for the 16 amp. Is this definitly the wrong type?

jon.

 

[jonny boy]

I have asked an electrician about fitting a 16 amp breaker for the saw but he just said I need a 16amp breaker and mentioned nothing about having to use a class C one.

jonathan.

 

[9fingers]

What make of MCB are you using. I'll see if I can look it up.

Class B are the normal domestic ones so maybe this is the B in your type number.

Bob

 

[jonny boy]

The make is HAGER

jon.

 

[9fingers]

I think they are Hager.

the one you want is like this

http://www.tlc-direct.co.uk/Products/HGNC116.html

Bob

 

[jonny boy]

Looks a reasonable solution to me Bob. Thanks for your help, you may have just saved me the cost of fetching out a spark. I'll also let the trician know how he could have mentioned this to me.

many thanks, i'll let you know how it go's.

jonathan.

 

[9fingers]

No probs Jon, only too happy to help.
I'm learning lots here about wood and if every now and then I can use my electrical/electronic knowledge to help then I feel I'm repaying the debt.

Bob

 

[davy_owen_88]

You can only fit a C rated MCB if your Zs (earth fault loop) are low enough to guarantee automatic disconnection in the event of an earth fault. You can NOT just go changing breaker ratings without testing first.

If your Zs are only just meeting the disconnection times for a type B breaker then upgrading to a C grade may leave live metalwork energised for sufficiently long enough for you to receive a fatal shock.

Maximum Zs for a B16 MCB - 2.87 ohms
Maximum Zs for a C16 MCB - 1.44 ohms

 

[jonny boy]

Davy, what is your take on why it keeps tripping then? I've just been reading some literature on MCB's and Bob's reply does tally with what i've found.

cheers,
jon.

 

[davy_owen_88]

I'm not saying that a c-rated breaker is not the answer because it is. What I am saying is that until you can prove that the c-rated breaker is going to disconnect the supply quick enough for you to survive a direct shock from the tablesaw during an earth fault you might want to hold off.

 

[jonny boy]

So basicall what you are saying is get a spark out to test it first? Are you? and how much would they roughly charge me?

cheers,
jon.

 

[davy_owen_88]

So basicall what you are saying is get a spark out to test it first? Are you? and how much would they roughly charge me?

cheers,
jon.

Testing is the only way to ensure that your Zs is low enough, but a few simple calculations can give you/me an idea.

If you can tell me your earthing type and the cable size and length between the consumer unit and the 16A outlet then I can give you a rough idea whether you can upgrade it safely.

 

[OJ]

If it's a new machine you shouldn't rule out a fault, get hold of a clamp meter and measure the actual load. As davy says, there is more to it than changing the mcb as that may be masking the real problem and cause you problems in the future. What is the rating on the motor plate? If you can give more details of the installation, then it would be easier to establish what is hapening. O

 

[jonny boy]

Davy,
The cable i'm using is 2.5 I think and the distance from the C/U to the 16 amp socket is no more than two metres, I dont know what you mean by the earthing type though. The motor is a three horse rated at 12.4 amps.

cheers,
jonathan.

 

[davy_owen_88]

There are 3 main types of earthing arrangement used in the UK.

TT - your main earth connection from the consumer unit runs to an earth rod that is driven into the ground somewhere around your house. This gives a very high Ze reading and as a result is impossible to meet disconnection times with MCB's alone so RCD's are used to meet this requirement. Your DNO has nothing to do with this type of earthing arrangement.

TN-S - the earth comes from the sheath/armour of your supply cable. Maximum permitted Ze is 0.8 ohms but this is not guaranteed as terminations corrode and screws work loose. If during testing the Ze is above that figure your DNO is obligated to remedy the problem.

TN-CS - the earth joins directly to the neutral in the cutout (the box that the service cable enters and also houses the main service fuse. The maximum permitted Ze is 0.35 ohms but again is not guaranteed as connections work loose etc. If during testing the Ze is above that figure then again your DNO is obligated to sort it out.

 

[jonny boy]

The C/U has been earthed to a copper pipe running next to it. If it helps, I can nip out to the garage and fetch the paper chart that was completed when the unit was installed. It's a new house only 4 years old and theres all the correct info written down in the C/U.

jon.

 

[davy_owen_88]

Hopefully, the CU isn't earthed to the copper pipe. An 'earth cable' running to water/gas pipes are actually main equipotential bonding conductors. This links all the accessible metal pipes within your house to the same potential as other earthed metalwork so that in the event of a fault there is not a potential difference between any two accessible metal parts so you can't get a shock.

If you could provide your earthing arrangement I can tell you if, under ideal conditions you are safe to upgrade to a C type breaker.