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British Computers: Power Connector Fuses - Values?

Posted: Sat Aug 05, 2017 11:03 am
by YuT666
Hi,

is there a "fuse list" of old british computers (Acorn/Tatung and others)? I'm talking about the fuses in the cable connectors. Most of them use 3A ... but i've heared about ones, that use 5A.

Cheers

Tom

Re: British Computers: Power Connector Fuses - Values?

Posted: Sat Aug 05, 2017 11:27 am
by 1024MAK
I don't know of any list as such.

With the UK electrical system, the fuse in the UK mains plug is primarily there to protect the flexible cord (cable) to the appliance. Of course, it also will provide protection for the appliance as well.

Most 1980s and 1990s "home" computers did not consume much power. Hence why a 3 Amp fuse is fitted in the mains plugs.

For example, the Acorn BBC Master 128 has a PSU rated at a power of 100 Watts. Acorn also helpfully also labelled it as rated at 0.5 Amps. [both these figures look to have been rounded up to "nice" numbers].

So work out the current from the power (in Watts), divide by the voltage. Watts / Voltage = current (in Amps). 100 / 230 = 0.434 Amps. So as the mains flex is 3A rated cable, a 3A fuse is the correct fuse to use from the standard values available.

Note that the available standard values are 3A, 5A and 13A. But you can also obtain 2A, 7A and 10A from some electrical suppliers.

It is also worthwhile pointing out that if the power of an appliance gives a current value near to a standard fuse value, normally the next highest rated fuse is used. Especially if the appliance will consume a much higher initial (surge) current.

Mark

Re: British Computers: Power Connector Fuses - Values?

Posted: Sat Aug 05, 2017 1:41 pm
by geraldholdsworth
In my PAT testing course, I was told that anything below 700W should have a 3A fuse fitted, and anything above 700W should have a 13A fuse fitted. What you fit if it is 700W, I can't remember what was said.

In addition, IEC leads (so-called 'kettle leads') should be fitted with 5A (and I then generally include Cloverleaf leads in with this), which then covers most IT equipment.

As Mark said, the fuse is there to protect the cable (as if the fuse is too highly rated, the cable will get warm and could burn out - effectively, the cable becomes the fuse). There are several sizes of cable, as measured as Cross Section Area (which should be written on the cable):
0.50mm sq: 3A
0.75mm sq: 6A
1.00mm sq: 10A
1.25mm sq: 13A
1.50mm sq: 15A

Cheers,

Gerald.

Re: British Computers: Power Connector Fuses - Values?

Posted: Sat Aug 05, 2017 2:49 pm
by 1024MAK
700W at 230V = 3.044A (rounded up)
700W at 240V = 2.917A (rounded up)

3A at 230V = 690W
3A at 240V = 720W

But as I said above, if the current is very near the value of the fuse, normally the next value up is used to prevent nuisance blowing.

In the UK the nominal mains voltage is now specified as 230V. But the actual value of the mains voltage can vary between by -6% to +10%. So between 216V and 253V. But in most towns and cities, 238V to 242V is typical.

The original intention was that UK electrical systems would indeed only use either 3A or 13A fuses in the mains plugs. So typically lighting, radios etc would have a 3A fuse in the plug. And anything that used a lot of power (heating, washing and cooking appliances) would use a 13A fuse. But that was before electronic devices and gadgets became popular, let alone home computers...

What is weird, is that before the 13A plug was introduced, we had 2A, 5A, 15A (to the BS 546 standard), and 5A, 10A and 15A [made by GH Scholes Ltd (Wylex) and used in many homes] plugs and sockets... So why the standard that introduced 13A plugs and sockets (which has been amended many times) only originally specified only 3A, 7A and 13A fuses (and then they dropped the 7A fuse from it), I'll never know :?

Mark

Re: British Computers: Power Connector Fuses - Values?

Posted: Sat Aug 05, 2017 3:36 pm
by dp11
People get very excited about plug fuses . For modern equipment you will find our friends in europe don't have plug fuses and just rely on the main fuse in the consumer. The plug fuse in the uk is designed to protect the cable and not the device. The only real cable fault you are going to get is a Live neutral/earth short. If the cable is reasonably short the fuse doesn't actually make much difference. You will find Kettles on sale with 0.75mm cable and fused at 13A as under cable fault conditions the 13A fuse will blow long before the cable become too warm.

A standard PAT person will fuse clover cables at 3A as the clover connectors is only rated at 2.5A.

If you want to be excited about fusing then there are a number of IEC leads which are made from 0.5mm cable so by the logic it should be fused at 3A. You also get 1mm cable IEC cables.

Re: British Computers: Power Connector Fuses - Values?

Posted: Sat Aug 05, 2017 3:39 pm
by dp11
Also people think a 13A will blown at 13.1A . Fuses and MCBs are a thermal device and take time to blow. A 13A fuse can pass 20A for quite some time before blowing.

Re: British Computers: Power Connector Fuses - Values?

Posted: Sat Aug 05, 2017 4:24 pm
by geraldholdsworth
dp11 wrote:You will find Kettles on sale with 0.75mm cable and fused at 13A

Most new kit sold is likely to have a 13A fuse in it, as it is cheaper to order a load at the same rating in bulk than to buy different ratings and actually fit the proper fuse.
dp11 wrote:A standard PAT person will fuse clover cables at 3A as the clover connectors is only rated at 2.5A.

Interesting...didn't know that. Can't remember if they were covered on my course, but I'll bear it in mind from now on.
dp11 wrote:If you want to be excited about fusing then there are a number of IEC leads which are made from 0.5mm cable so by the logic it should be fused at 3A. You also get 1mm cable IEC cables.

Yep - there is a level of judgement at the time of testing. What I quoted were only general guidelines for the bulk of leads/equipment.

Re: British Computers: Power Connector Fuses - Values?

Posted: Sat Aug 05, 2017 6:47 pm
by dp11
Below is a picture of some fake fuses I've found during PAT . The last fuse I confiscated from a PAT person who bought them from ebay.

fakefuses.png


Who can list all the reasons these are fake?

Re: British Computers: Power Connector Fuses - Values?

Posted: Sat Aug 05, 2017 7:06 pm
by topcat96
dp11 wrote:Who can list all the reasons these are fake?


Genuine fuses have some form of filler material such as sand in the main fuse body to help stop a fuse exploding and causing more damage.

Gaps between the metal endcap and the body are a dead giveaway too.

Fuse wire should not be poking out of the endcaps as genuine ones have an internal fitting direct to the cap itself

250v shown on the fuses

A BS1362 fuse must have the manufacturer's name or identifying mark.

Re: British Computers: Power Connector Fuses - Values?

Posted: Sat Aug 05, 2017 7:06 pm
by jgharston
dp11 wrote:Below is a picture of some fake fuses I've found during PAT . The last fuse I confiscated from a PAT person who bought them from ebay.
fakefuses.png
Who can list all the reasons these are fake?

No sand inside the cartridge.
Fusewire too thick
Fuse is actually a solid conductor (think a painted 1/4" nail!)
No colour band/tiger teeth
Fuses are to BS1362 not BS1363, BS1363 is the plug/socket
Cartridge body too white, made from wrong sort of ceramic
I think on that 5A one I can see the fuse wire poking out of the cap, instead of it being welded internally
No kitemark or BSI mark
Is that 13A one too short?

ooo. how many more can I spot? :)

Re: British Computers: Power Connector Fuses - Values?

Posted: Sat Aug 05, 2017 7:54 pm
by YuT666
A big thanks to you all ...

This is an interesting matter (for me).

Our usual fuses (here in germany) have the sizes 5x20mm and 6x30mm and i can't remember any fuses in 99% of our plugs.

The main fuses in the UK seem to have 6x25mm. My Micro/Master/AcornA3000/Tatung are using 3A ... but a found a loose 6x25 fuse with 5A. Maybe for the Dragon32? The Dragons plug is empty and i can't find an info inside the plus.

And i really don't know exactly, which residual current operated circuit breaker (i hope, that this translation is right) you usually use in the UK ... we mainly use 16A ones here.

Re: British Computers: Power Connector Fuses - Values?

Posted: Sat Aug 05, 2017 9:05 pm
by 1024MAK
YuT666 wrote:A big thanks to you all ...

This is an interesting matter (for me).

Our usual fuses (here in germany) have the sizes 5x20mm and 6x30mm and i can't remember any fuses in 99% of our plugs.

The main fuses in the UK seem to have 6x25mm. My Micro/Master/AcornA3000/Tatung are using 3A ... but a found a loose 6x25 fuse with 5A. Maybe for the Dragon32? The Dragons plug is empty and i can't find an info inside the plus.

And i really don't know exactly, which residual current operated circuit breaker (i hope, that this translation is right) you usually use in the UK ... we mainly use 16A ones here.


BS 1362 Fuse dimensions (as used in UK 13A plugs):
Dia - 6.3mm +0mm, -0.025mm (1/4 inch approx)
Length - 25.4mm +0.8mm, -0.4mm (1 inch)

We also use 5mm dia x 20mm length and 6.3mm (1/4 inch) dia x 32mm (1 and 1/4 inch) length in equipment and appliances.
Occasionally (normally in industrial equipment) are found 5mm x 25mm fuses, which have a coloured indicator cap (which falls away when the fuse blows).

This site shows various fake UK mains plug fuses.

I don't have a Dragon 32, so don't know the answer, but I would have thought it also would have used a 3A fuse.

In older homes where the consumer unit has not been replaced, either rewireable fuse holders (using fuse wire) are used, or cartridge fuses are used. In modern homes, or where a consumer unit has been replaced, we have MCBs (Miniature Circuit Breakers). MCBs have two methods of detecting a short circuit or overload (thermal and magnetic). Typical ratings found in modern units are 3A (door bells, smoke / fire alarms, alarm systems etc), 6A (lighting circuits), 10A (industrial lighting), 16A (water heaters, radial spurs), 20A (radial spurs), 32A (ring main circuits, cookers), 40A (showers, cookers), 45A (showers, cookers), 50A (high power showers). For protection against insulation failure (including electric shock) RCBOs (Residual Current Breakers with Overcurrent protection) are available in most of the ratings listed above (except for 3A). These trip just like the MCBs, but also if there is an imbalance in the current between the line (live) and the neutral wires (which indicates an insulation failure / earth fault).

Note that where fuses or fuse wire is used in a consumer unit, the fuse values are 5A, 15A, 20A, 30A. Some MCBs designed to fit in consumer units what were originally designed for fuses have the same ratings as the fuses they replace.

Our 13A power outlets / sockets are normally wired to a 30A or 32A ring main circuit. Hence why U.K. 13A plugs have a fuse included.

Mark

Re: British Computers: Power Connector Fuses - Values?

Posted: Sun Aug 06, 2017 9:53 am
by geraldholdsworth
1024MAK wrote:Typical ratings found in modern units are 3A (door bells, smoke / fire alarms, alarm systems etc), 6A (lighting circuits), 10A (industrial lighting), 16A (water heaters, radial spurs), 20A (radial spurs), 32A (ring main circuits, cookers), 40A (showers, cookers), 45A (showers, cookers), 50A (high power showers).

I think my house has been wired incorrectly, as I cannot find the breaker for the shower - I suspect that it has been wired into one of the two ring mains.

Re: British Computers: Power Connector Fuses - Values?

Posted: Sun Aug 06, 2017 12:13 pm
by jgharston
geraldholdsworth wrote:
1024MAK wrote:Typical ratings found in modern units are 3A (door bells, smoke / fire alarms, alarm systems etc), 6A (lighting circuits), 10A (industrial lighting), 16A (water heaters, radial spurs), 20A (radial spurs), 32A (ring main circuits, cookers), 40A (showers, cookers), 45A (showers, cookers), 50A (high power showers).
I think my house has been wired incorrectly, as I cannot find the breaker for the shower - I suspect that it has been wired into one of the two ring mains.

A common method of installing an electric shower was to add a additional one-way fuseway specifially for the shower and feed it from the supply tails like this.

Years ago I had to rewire my brother's kitchen as the whole house was on one ring main with the cooker plugged into the ring, and when the tumble drier, cooker and kettle was on, when the fridge came on the fuse blew. I gave the kitchen its own ring and the cooker its own radial.

Re: British Computers: Power Connector Fuses - Values?

Posted: Sun Aug 06, 2017 12:18 pm
by jgharston
jgharston wrote:Fuse is actually a solid conductor (think a painted 1/4" nail!)

Screams, runs and hides!
Image