16, 32 and 63 as a rating

[Cookie]

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:

https://imgur.com/a/trtK7pI

View: https://Upload the image directly to the thread.com/a/trtK7pI


The nominal voltages which go along with them:

https://imgur.com/NFO74Tp

View: https://Upload the image directly to the thread.com/NFO74Tp

 

[M-Ty]

It is something to do with the ISO using the Renard series as preferred values.

 

[Cookie]

It is something to do with the ISO using the Renard series as preferred values.

Renard? Why are these values preferred?

 

[Julie.]

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 )

 

[Cookie]

Wasn't the UK even 250 volts at one point? I like that number!

 

[Lucien Nunes]

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.

 

[static zap]

Did some Voltage warnings signs diminish as well ?

 

[pc1966]

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]

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.

 

[Cookie]

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.

 

[Cookie]

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]

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

 

[Cookie]

Good point.

But still not so sure about MCBs.

 

[Cookie]

Did some Voltage warnings signs diminish as well ?

250 volts is a nice even number to work with. I think Australia was 250 volts at one point as well.

 

[ElectroChem]

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.