RCD Diversity?

[kentster]

Hi All

Newbie so be gentle on me. I'm training to be a spark at the moment (early stages) and have a simple question on diversity. I know you can use diversity in regards to overcurrent protection but can you also apply to diversity to RCDs?

Was in B&Q walking round and noticed a massive display of consumer units fully populated with MCBs (1 x 40, 4 x 32, 2 x 16 & 3 x 6A) with the usual 100A main switch but with 2 x 30mA 63A RCDs.

Does this mean that you apply diversity to work out what circuits you can protect by each one?

 

[tomxhug]

its probably a double split board, it needs to be split to avoid nuisancance tripping of the RCD

e.g.
RCD 1 up ring, down lights, Shower,
RCD 2 DOwn RIng, up lights, Cooker

if wired right only one RCD will trip, If wired wrong youll never get anything working.

 

[nickblake]

Its a Duel RCD board ,17th edition as commonly known these have been introduced to comply with the current regulations ,and should be laid out as tomxhug has said as all circuits should now be protected by an RCD unless some areas of the regs can be reached ie installed in a earth metal wiring system or all deeper that 50mm from the surface etc , its a cheaper option as these boards can cost as little as 59.99 where if you were to use RCBO a combination of MCB /RCD a 10 way board could cost you £320.00

 

[kentster]

Yep it was a split load board but still confused slighty....As tomxhug put above: -

RCD1 up ring (32A) + down lights (6A) + 9.6kW Shower (40A) = 78A total
RCD2 down ring (32A) + up lights (6A) + Cooker (32A) = 74A total

So I know that you can use diversity to allow the total of 152A to run through the 100A main switch but is the 78 & 74A allowed to be protected through each of the 64A RCDs using the same diversity calculations?

 

[telectrix]

personally, i would say yes. the point is not to exceed the 63A load capacity of each RCD. As diversity allows for the fact that not every circiut will be pulling it's maximum current at any one time, then the same should apply for the 4/5 circuits supplied by each RCD. You are not likely to be using the kettle in the kitchen at the same time as the teasmade in the bedroom, or the shower at the same time as the cooker. Bit simplified , I know, but that's my basic theory.

 

[tomxhug]

yet another problem with not having 3 phase in the modern house, and giving customers the option of buying from large DIY superstores!

 

[malcolmsanford]

Yep it was a split load board but still confused slighty....As tomxhug put above: -

RCD1 up ring (32A) + down lights (6A) + 9.6kW Shower (40A) = 78A total
RCD2 down ring (32A) + up lights (6A) + Cooker (32A) = 74A total

So I know that you can use diversity to allow the total of 152A to run through the 100A main switch but is the 78 & 74A allowed to be protected through each of the 64A RCDs using the same diversity calculations?

A quick rule of thumb your 2 RCD are basically 2 Installations. So on one 78amps multiply that by 0.4 and you get 31.2 amps. This 0.4 rule for domestic installations is a rule of thumb that the DNO use when working out diversity loadings.

 

[telectrix]

i uisually go at 0.5. it's easier if you not got a slide rule handy. 78/2=39, all in me 'ed. keep it simple and err on the side of caution.

 

[malcolmsanford]

i uisually go at 0.5. it's easier if you not got a slide rule handy. 78/2=39, all in me 'ed. keep it simple and err on the side of caution.

True old mate and your on the side of caution as well. Just old habits as it was always 0.4 domestic, 0.6 commercial and offices and 0.8 industrial easy to remember lol

 

[kentster]

Thanks everyone for answering, it makes sense now, else I couldn't see the point of making a consumer unit with 63A RCDs for a standard domestic installation. Time to stop evenings in the pub to read the BRB for another few weeks of revision...

 

[Sparky Ninja]

Diversity for the standard arrangement of a household is 100% of current demand of the largest point of utilization, +40% of current demand of every other point of utilization. Irrelevant of RCD number.

 

[Gavin121]

I assume the diversity factors are given in the red book or somewhere else perhaps?

 

[kentster]

Gavin - Gonna have my head in the BRB & OSG tonight anyway so if I find them in either i'll post the page numbers on here (unless someone beats me to it)

 

[malcolmsanford]

Diversity for the standard arrangement of a household is 100% of current demand of the largest point of utilization, +40% of current demand of every other point of utilization. Irrelevant of RCD number.

Diversity is horses for courses and appendix 1 in the osg states several times that table 1B is purely a guide, that in the 6 paragraphs written it shows 2 methods, and dosen't preclude the use of any other method, and in nearly every paragraph it puts the onus onto the designer/sparks.

Diversity is nigh on impossible to calculate for 1 house may decide to fit 6 3kw heaters onto the 3 ring final circuits but next door only uses their ring final circuits for televisions and computers.

I'm all for Table 1B for fixed load appliances and such as that is a constant, but even then a cooker in a house that as mum, dad and 6 children will be on far more than a house where someone lives on their own.

If you used the Table 1 method for calculating the OP installation it would come out at 104 amps for 3 ring final circuits, 2 lighting circuits and a shower, using the rule of thumb method of 0.4 it comes out to 59.2 amps. As this installation as just 6 circuits the DNO would fit maximum 80 amp and most likely if a 60amp was fitted it would never fail.

I always found the rule of thumb methods quite reliable and as the OSG say it's ultimately down to the designer

 

[Sparky Ninja]

Diversity is horses for courses and appendix 1 in the osg states several times that table 1B is purely a guide, that in the 6 paragraphs written it shows 2 methods, and dosen't preclude the use of any other method, and in nearly every paragraph it puts the onus onto the designer/sparks.

Diversity is nigh on impossible to calculate for 1 house may decide to fit 6 3kw heaters onto the 3 ring final circuits but next door only uses their ring final circuits for televisions and computers.

I'm all for Table 1B for fixed load appliances and such as that is a constant, but even then a cooker in a house that as mum, dad and 6 children will be on far more than a house where someone lives on their own.

If you used the Table 1 method for calculating the OP installation it would come out at 104 amps for 3 ring final circuits, 2 lighting circuits and a shower, using the rule of thumb method of 0.4 it comes out to 59.2 amps. As this installation as just 6 circuits the DNO would fit maximum 80 amp and most likely if a 60amp was fitted it would never fail.

I always found the rule of thumb methods quite reliable and as the OSG say it's ultimately down to the designer

I don't disagree with what you think, I just quote from the IEE everytime.

The diversity from the OP cannot be accuratley assessed anyway as the rated currents cannot be calculated.

 

[pushrod]

Anyone know how quickly an rcd cooks if you exceed it's rated capacity. Have you got much leeway and for how long?
Also what is the most likely damage? does it just take out the trip and leave you thinking it is still ok or does it stop it conducting altogether?