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Why are 16, 32 and 63 amps used as an MCB rating instead of 15, 30 and 60 amps? Best theory that I can come up with was when the UK switched to 230 volts this was to make up for the drop in voltage but have noting to back that up.

Here are the ratings I'm familiar with:

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The nominal voltages which go along with them:

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  • Thread Starter Thread Starter
  • #3
It is something to do with the ISO using the Renard series as preferred values.

Renard? Why are these values preferred?
 

Julie.

Arms
Esteemed
The UK never changed voltage.

We had 240/250V as nominal with an allowance to go down a bit and over a tiny bit.

Now we have 230V with an allowance to go up a bit and down a tiny bit.

They are essentially the same voltage range, so in terms of practical changes the UK didn't change.

R'10 preferred values are now used right throughout the industry, unfortunately ratings have only partially moved (6A doesn't fit the series it should be 6.3A, 45A shouldn't fit only fits in R20, R'20, R''20 and so on series )
 

Lucien Nunes

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Many different voltages were in use in different areas, from 100 to 250, even different frequencies (and DC) before the national grid standardised all new supplies from around 1931 onwards. The highest voltage in normal use was 250V, so most single-phase accessories were rated for that voltage, and most 3-phase equipment for 500V.

The majority of supplies were 200V or higher, therefore 5A was usually sufficient for 1kW and the 5A multiples approximately corresponded to the kW rating of typical loads.

Typical domestic applications were:
5A - lighting, small outlets
10A - heating, fell out of favour in the 1930s
15A - heating, water heating (stored), large domestic outlets
20A - radial circuit for small numbers of BS1363 outlets and FCU-attached appliances (after 1947)
30A - cooking, ring final circuits for BS1363 outlets (after 1947), main fuses for small installations
45A - large cooking, submains, showers (not then popular)
60A - main fuses

The UK was quite old-fashioned in terms of OCPD choice, we continued to favour rewireable fuses when Europe was increasingly using HRC cartridges. Fuse wire was sold in the above ratings and these were carried over to HRC fuses when we started to get into those. You can still buy cards of fuse wire in hardware stores, with the most popular domestic ratings, 5, 15 & 30A and sometimes 20A.

As the popularity of MCBs increased the first really successful product series also used the multiples of 5A e.g. Crabtree C50 (that's a model number, not a rating), Wylex plug-in and NB types, MK LN / Sentry and other early BS3871 models. We migrated to the Renard number ratings with the rest of Europe in the 1980s. Series that were current at the time existed in both flavours. E.g. MK Sentry Mk.1 can be found with 5A, 6A, 10A, 15A, 16A etc ratings. Many products such as industrial switchgear also made the shift (30-32A, 60-63A etc) with the same basic products being rebadged with the new rating, perhaps with a styling facelift.
 

pc1966

Arms
Esteemed
Renard? Why are these values preferred?
They have (approximately) equal ratios between values.

Like the resistor series (e.g. E12 going 1.0, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, 8.2) where the steps are based on the same percentage tolerance.
 

davesparks

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when the UK switched to 230 volts this was to make up for the drop in voltage but have noting to back that up.
The UK didn't switch to 230V anywhere but on paper, the national grid didn't magically change voltage overnight, all of the final substation transformers stayed right where they were with a 250/433 output.

Also increasing fuse rating to coincide with a reduction in voltage wouldn't make sense, generally the overall effect would be a slight reduction in load current for the majority of cases.
 
  • Thread Starter Thread Starter
  • #10
Also increasing fuse rating to coincide with a reduction in voltage wouldn't make sense, generally the overall effect would be a slight reduction in load current for the majority of cases.
I see it like this-

15 x 240= 3,600 va

16 x 230= 3,680 va

Although that assume the load will actually be rated 230 vs 240 volts.
 
  • Thread Starter Thread Starter
  • #11
They have (approximately) equal ratios between values.

Like the resistor series (e.g. E12 going 1.0, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, 8.2) where the steps are based on the same percentage tolerance.
What would be the advantage of this?
 

pc1966

Arms
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What would be the advantage of this?
Well is you make E12 resistors with a +/-10% tolerance then every resistor falls inside one of those bands :)
 

ElectroChem

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Arms
Good point.

But still not so sure about MCBs.
The way it was explained to me is that the series of values aids in coordination and discrimination between breakers.

If you take the series
16, 20, 25, 32, 40, 50, 63, 80, 100
then from each point, if you go up TWO steps, then that value is ~1.6x where you started, and your downstream breaker should now reliably trip before the upstream.
 
  • Thread Starter Thread Starter
  • #16
The way it was explained to me is that the series of values aids in coordination and discrimination between breakers.

If you take the series
16, 20, 25, 32, 40, 50, 63, 80, 100
then from each point, if you go up TWO steps, then that value is ~1.6x where you started, and your downstream breaker should now reliably trip before the upstream.

IF you're in the thermal region, right?
 

pc1966

Arms
Esteemed
Yes, the thermal curves provide selectivity, but once you hit the magnetic trip there is very little.

But for fuses it is generally OK for BS88 style at a 1:1.6 sort of ratio, but to be sure you would normally go for a 1:2 ratio. For example, if you have a 80A DNO fuse then a 40A fuse-switch to feed something (out building DB, whatever) then wherever happens on the end of that won't take out your main supply.

Getting selectivity MCB to fuse is hard as well, but often if you have a 1:2 ratio and B-curve you get a usable level, e.g. 20A B-curve & 40A fuse is going to be to PFC of around 900A which, depending on cable length, might never be reached so the fuse lives past a MCB trip on a hard fault.
 
  • Thread Starter Thread Starter
  • #18
Yes, the thermal curves provide selectivity, but once you hit the magnetic trip there is very little.

But for fuses it is generally OK for BS88 style at a 1:1.6 sort of ratio, but to be sure you would normally go for a 1:2 ratio. For example, if you have a 80A DNO fuse then a 40A fuse-switch to feed something (out building DB, whatever) then wherever happens on the end of that won't take out your main supply.

Getting selectivity MCB to fuse is hard as well, but often if you have a 1:2 ratio and B-curve you get a usable level, e.g. 20A B-curve & 40A fuse is going to be to PFC of around 900A which, depending on cable length, might never be reached so the fuse lives past a MCB trip on a hard fault.

Good to know.

RK-1 and RK-5 fuses will achieve 100% selective coordination up to 200,000 amps provided a 2:1 ratio.
 
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