16 amp help wanted!!

[jonny boy]

What I did want to sak you Davy is, The breaker i've bought is made by Wylex whilst the C/U is a hager. The breaker will fit in just fine but does it matter if it's a different make or should I have waited and sent for a hager one?

cheers,
jon.

 

[davy_owen_88]

The page you are looking at is the schedule of test results. This shows the test results for each final circuit. The PSCC is a supply characteristic as it is measured at the origin of the supply (ie right after the meter).

Something like this:

 

[davy_owen_88]

What I did want to sak you Davy is, The breaker i've bought is made by Wylex whilst the C/U is a hager. The breaker will fit in just fine but does it matter if it's a different make or should I have waited and sent for a hager one?

cheers,
jon.

It's best to stick to one manufacturer as mixing and matching voids certain warranties as parts from different manufacturer's have not been tested within that particular consumer unit. If the MCB makes good contact with the bus-bar then there is little to go wrong, but it must align properly and make a tight connection otherwise there is a risk of overheating.

 

[jonny boy]

Davy, the sheet I have looks nothing like the one shown, it's a photo copied chart listed as form WR4 and called installation schedule (including test results).
If I isolate the C/U and fit the breaker, can I do any harm to anything or do you think the 16amp class C 10kA breaker will be OK?

 

[jonny boy]

I'm going to have another chat with the electrician who does all of our wiring at work and see if he'll come and fit it all correctly and in the meantime, order the same breaker as the hager one that matches all the others in the C/U. For the sake of four quid I think it might be best.
(and it all seemed straight forward yesterday)

jonathan.

 

[davy_owen_88]

On the assumption that the correct procedures were carried out when the electrician tested everything then there is nothing that can go drastically wrong. The only problem is, I hate making assumptions when peoples lives and houses are at stake :roll:

If your PSCC is higher than the breaking capacity of the MCB's then the responsibility lies with your service fuse to cut off the supply before damage can occur. If the service fuse isn't capable of breaking the fault then the next line of defence is the 400A HBC fuse in the transformer... When 96kW flows through standard domestic wiring things start to burn, and pretty damn quickly too which is why you see it is very important to know these things. Unfortunately your spark didn't measure the PSCC and also didn't seem to fill out the details for the service fuse so without a working crystal ball I can only hope everything is fine.

Does the forms you have resemble anything shown here:

IEE Forms

Particularly the 'Nature of Supply Parameters' on page 6.

 

[jonny boy]

Thanks again for your advice Davy, I'm not needing to use the saw immedietly so I wont fit it just for the sake of trying. I'll get the hager breaker and ask the spark to come and have a go but how confident are you that the problem has been with using a standard class B and not the motor rated class C ?

 

[jonny boy]

Yes Davy, the form is identical to the schedule of test results one.

 

[davy_owen_88]

It's almost definately down to the B rated breaker. B-rated breakers have an instant trip within the region of 5x their rated current. So your 16A B type fuse will trip within 100ms when it detects currents of 80A which is more than possible with a 3HP induction motor. C-rated breakers have an 'instant trip' of 10x so requires 160A (hence the reason your Zs needs to be half to allow twice as much current to flow for it to trip during earth faults within the same time limit).

Regarding changing the MCB. There is nothing stopping you changing the MCB today yourself. I was merely pointing out the meaning of the 10kA rating and what it's point was. Unless you live very close to a substation it is unlikely that your PSCC is going to be higher than the breaking capacity of your service fuse unless major work has been done to the supply system since the cable and fuse was installed by the DNO. I just don't like telling people that everything is fine when I can't guarantee that it is. Just make sure you don't go shorting any conductors until you have the PSCC measured and the breaking capacity of the service fuse confirmed. :wink:

 

[jonny boy]

Thanks again for your help Davy and in worst case senario if the Ze was to be higher, then what would happen?

 

[davy_owen_88]

Ze has nothing to do with PSCC. Short circuits occur between phase and neutral whilst earth faults occur between phase or neutral and earth. If your overcurrent protective devices can break a P-N fault then they can break an earth fault as more current will flow in a short than during an earth fault.

The reason I mentioned Zs again was because I mentioned the current requirements for an instant trip between B and C type MCB's and that explains why the Zs needs to be lower for a C-type because for the MCB to trip 'instantly' twice as much current needs to flow so the Ze needs to be half.

 

[jonny boy]

The thing that is worrying me is, can I end up doing any damage to the existing supply which would result in Having to get a sparky out to repair my mistake.

 

[davy_owen_88]

If you are careful when you wire it up and make sure that all connections are tight, no two different conductors can make contact, all bare earth conductors are sleeved and no cables are damaged then no, you can't do anything that can cause any damage.

If you are confident in your electrician - i.e that he did measure the PSCC and check the breaking capacity and found to be sufficient but just forgot to write it down, then even if you did make a mistake the worst that will happen is the fuse will blow. If the service fuse blows your DNO have a requirement to replace it within (IIRC) 3 hours. The service fuse shouldn't blow unless your PSCC is particularly high though, which as mentioned will only really be the case if you are very close to a substation.

 

[jonny boy]

And if I was using the wrong breaker, is there any possibility of damage to the machine motor?

I dont want to turn the electric off tonight because we are all watching T.V and cooking the tea but i'll have a go first thing in the morning and see if it's all systems go. But I think i'll still take your advice and send for a hager breaker to match all the others.

 

[davy_owen_88]

You can't damage the motor with an MCB. If it is made for a 16A supply and you use a 16A breaker with suitable cables then there is nothing you can do to damage it.

 

[jonny boy]

Thanks, I'm off to get some tea now but thanks again for the help, i've learnt an awfull lot from you over this problem and will have hopefully resolved the situation. Fingers crossed for the morning and i'll let you know how it goes.

Thanks for your time,
JONATHAN.

 

[jonny boy]

Hello, back again and after a chat to the electrical engineer who does various bits for us at work, he's gonna come over one day in the week and make sure it's done correctly. He say's if I fit the 16amp socket and an isolator to the wall where I want them, he'll come and just connect it for £20. I think it's worth the peace of mind knowing it's been done by someone who knows what their doing. I'm also certain that if I did it, and God forbid the house burnt down, the insurance wouldn't be worth the paper it's written on. I'll come back when the connection is done.

cheers,
jonathan.

 

[jonny boy]

Hello,
It's finally done! I had the 16 amp class C breaker fitted and guess what....... it only went and tripped again as soon as it was switched on, not to worry said the spark, stick a 20amp in and bingo, my Unisaw is a runner and it has one hell of a kick on start up as well. When I work out how to post photo's, i'll get some on because it's a grand edition model in white and looks exellent.
Thanks to everyone who gave help and advice and I got there in the end!

cheers,
jonathan.

 

[9fingers]

Glad to hear that you can make sawdust once again.
Bit of a shame in one way though because it does mean I have no chance now in offering you a few beer vouchers to take the old unisaw away and tell you to buy something else that will run on a 13 amp plug!

Regards

Bob

 

[jonny boy]

Thanks for the help you gave Bob, is it no suprise to you that it would still trip a 16amp breaker? even if it was a class C one?

jon.