second opinion

[securespark]

Removed from OSG in AMD 1??

I hadn't caught up with that update yet...

 

[JohnW2]

Removed from OSG in AMD 1?? I hadn't caught up with that update yet...

I haven't seen the new OSG yet but, in Amendment 1, the statements in ('Informative') Appendix 15 about floor area change from, say:

As a rule of thumb the floor area served has been limited to 50m²

(which I reckon is weak enough, partcularly given the "has been")

TO

Historically the floor area served has been limited to 50m²

So, as I said, no longer even a 'rule of thumb', and now essentially just a statement of historical fact. If I were in the game, I'd certainly see no reason to regard failure to 'comply' with that statement about history as being a 'deviation from BS7671' - what do you think?

Kind Regards, John

 

[ban-all-sheds]

Probably just had 16A's to hand would be my guess.

My guess could well be that the guy didn't really understand the principles of ring finals and put 16s in because 16 is half of 32.

 

[JohnW2]

My guess could well be that the guy didn't really understand the principles of ring finals and put 16s in because 16 is half of 32.

That seems pretty likely. I'm also a little surprised how often one hears these stories about a ring final being converted to two radials as a lazy (or 'temporary') solution when a fault arises in the cable - why is it that such circuits seem so often to develop cable faults roughly in the centre of the ring?!

In reminds me that I've often wondered why it was that 5, 15 and 30A fuses metamorphosised into 6, 16 and 32A MCBs. Anyone know why they adopted ratings which were very slightly higher than the corresponding fuses?

Kind Regards, John.

 

[elrobbo82]

My guess could well be that the guy didn't really understand the principles of ring finals and put 16s in because 16 is half of 32.

also probably correct. just seems daft that the kitchen has far more outlets than just half of the ring with a larger demand and this spark thought it was a good idea to protect the kitchen radial with a 16A mcb. as stated not unsafe just bad design due to nuisance tripping.

 

[starspark]

Okay, what do you propose the design current to be then?

i would like to see a 4mm radial protected via a 32A mcb ideally for the appliances in this particular property. 2.5mm 20A would be ideal for the living room as it has a low load demand. but as a ring had already been installed i would prefer to reinstate the ring and do away with the radials.

So.... thanks for all that info!!!
Anyway back to the question what is your proposed design current??

 

[mfarrow]

in Amendment 1, the statements in ('Informative') Appendix 15 about floor area changed

This was done in the corrigendum to the BRB back in 2008.

Time to get a pair of scissors and a pritt stick out.

 

[elrobbo82]

starspark
it can be tricky calculating the max demand on this circuit with out knowing the exact loading on each of the "utilization points". maybe you could steer me in the right direction and show how you would calculate design current for the kitchen radial taking into account it has four high loading appliances (washer, dryer, kettle and fridge) and 3 double sockets. my own calcs using diversity would place above 16A how about you.

 

[JohnW2]

This was done in the corrigendum to the BRB back in 2008. Time to get a pair of scissors and a pritt stick out.

So it was - I'd forgotten that So, it hasn't been a 'rule of thumb' since 2008. No need for the pritt stick,though - the BGB has saved me from the need to do that!

Kind Regards, John.

 

[RMS]

i know that this is safe but i am unimpressed with the possible load on the kitchen radial (2.5mm 16A 60898) as 2 appliances could overload the circuit.

Strange, now your saying that tripping has occurred due to overload.

Also, strange that the customer will call you out to replace a faulty socket but will happily live with having to reset the protective device from time to time. (Some people are strange).

So you are obviously aware of the faulty cable. Have you tested the cable and established the fault?

Why don't you clamp an ammeter in circuit and run some appliances and see what sort of load the circuit requires?

 

[elrobbo82]

Strange, now your saying that tripping has occurred due to overload.

Also, strange that the customer will call you out to replace a faulty socket but will happily live with having to reset the protective device from time to time. (Some people are strange).

So you are obviously aware of the faulty cable. Have you tested the cable and established the fault?

Why don't you clamp an ammeter in circuit and run some appliances and see what sort of load the circuit requires?

i have rectified the fault today as when i tested the damaged cable and got a short of line to earth. customer had placed a picture on the wall. took the nail out and it cleared. ran in new leg using surface trunking and re installed the ring.

amazing that something so simple of checking the demand with appliances on with a clamp meter just slipped my mind.

 

[elrobbo82]

trunking looks ok due to being low down just above skirting board. fridge is not a high load appliance you are correct. clamp meter would have been a good idea to check.

thanks for the info what about the question with regards to how you would calculate the design current?

 

[JohnW2]

it can be tricky calculating the max demand on this circuit with out knowing the exact loading on each of the "utilization points". maybe you could steer me in the right direction and show how you would calculate design current for the kitchen radial taking into account it has four high loading appliances (washer, dryer, kettle and fridge) and 3 double sockets. my own calcs using diversity would place above 16A how about you.

You might be interested to look at the article entitled "Amendement No. 1 2002 to BS7671 and ring currents" in the Spring 2002 edition of Wiring Matters. Despite the rather uninspiring title, this article includes a very interesting analysis of the loading on a kitchen ring when a washing machine, dishwasher and tumble dryer are started on their cycles simultaneously and an electric kettle used twice during the period the other machines are running. It concludes that this loading is (just about!) OK for most of the time with a 32A ring - but a 16A or 20A radial would clearly not be adequate for that loading.

Kind Regards, John.