Over-current protection & the fusing factor.

[david911cockburn]

Hi Guys,

I don't think we can say that the Regs. have ever been wrong, some of the advice that has been given showed a bias towards changing from fuses to circuit breakers, but I wouldn't want to 'split hairs' over that.

The basic advice has always been that small overloads of a long duration should be avoided.

In the past we where provided with a number of different fuse carriers and a huge amount of choice of fuse elements or fuse wire, but how many people actually sat down and worked out that (given the advice above) a domestic 30amp ring main should be protected by a 30amp fuse carrier containing 20amp fuse wire?

 

[ban-all-sheds]

how many people actually sat down and worked out that (given the advice above) a domestic 30amp ring main should be protected by a 30amp fuse carrier containing 20amp fuse wire?

Only those who didn't understand it all I suppose.

Anybody who did sit down, work it out, and come to that conclusion was obviously an incompetent buffoon who should never have been allowed anywhere near the task of circuit design.

 

[david911cockburn]

Go on, explain yourself.

 

[ban-all-sheds]

Explain myself?

In what way?

If you think I'm going to type in all of the explanations and regulations surrounding the calculation of cable sizes so that you can then see that there's nothing in them which arrives at needing a 20A fuse then you're going to be sadly disappointed.

But don't you agree with me?

I thought you were postulating that someone might sit down and, given the above, incorrectly work out that a domestic 30 amp ring main (sic) should have a 20A fuse?

Don't you think that anybody who could get it so wrong would be an incompetent buffoon who shouldn't be having anything to do with circuit design?

 

[david911cockburn]

Shed,

A 30amp ring main must be protected by a 30amp fuse carrier containing 20amp fuse wire.

I didn't say 20amp fuse.

Is this a bit too complicated for you?

 

[ban-all-sheds]

A 30amp ring main must be protected by a 30amp fuse carrier containing 20amp fuse wire.

1) Why?

2) Please show your reasoning.

3) Please show how such a circuit would comply with BS 7671.

4) Please explain how if it had 20A fuse wire protecting it it could be called a "30amp ring main".

I didn't say 20amp fuse.

My apologies - I didn't realise that you wouldn't be able to understand that by "20A fuse" I was referring to "a 30amp fuse carrier containing 20amp fuse wire".

 

[ban-all-sheds]

Hi Sheds,

I don't wish to sound paranoid but the IET have a history of 'editing' what I have to say in order to make it un-readable, but I will give it some thought.

Well - I see that you did do it.

http://www.theiet.org/Forums/forum/messageview.cfm?catid=205&threadid=40756

Was the general opinion there that you were talking b****cks due to the IET editing what you wrote in order to make it unreadable, or because you were talking b****cks?

 

[ericmark]

I have in resent years seen a move back to fuses as MCB's are too slow. Fitting a MCB before a semi-conductor relay was the first time I realised the major problem with a MCB.

It supplied a bank of heaters which were being switched many times a shift and mechanical contactors just did not last.

So the mechanical contactor was replaced with a semi-conductor version. So started a string of problems. If a heater went short circuit it would every time take out the relay. The MCB was replaced with a fuse carrier and semi-conductor fuses were used this stopped that problem.

Because three phase it was realised we could lose just one phase so an anti single phasing trip was added.

Then it was realised it now did not comply with safety laws as E-Stop was on control circuit to semi-conductor device so a E-Stop relay was added.

This of course will have been experienced by many electricians on how so called improvements have caused a series of events before the improvement has really worked.

We now have the same problem with or domestic supply. Since moving from the 30A fuse to a 32A MCB it is common for the MCB to trip before the 13A fuse will blow. This was even more of a problem with lights. Never knew a 5A fuse to go on lighting circuit. But the 6A MCB would regularly trip as a bulb popped. Bulbs were supposed to have built in fuses which should blow if gas in bulb ionised. Not sure if trip going was due to cheap bulbs without built in fuse or speed of MCB?

The big problem is we all I hope understand type B, C, and D MCB's but let through current of the fuse is another story. There are so many different types of fuse that most of us if designing need to look up the parameters every time.

I had considered returning to a fuse for lights but now with discharge lighting all changed again.

 

[bernardgreen]

So the mechanical contactor was replaced with a semi-conductor version.

Been there, got the T shirt and the big pile of blown solid state switches.

This of course will have been experienced by many electricians on how so called improvements have caused a series of events before the improvement has really worked.

In many cases there is no improvement and the costs incurred in getting back to the original quality but with the solid state switch can be more than the cost of a ten year supply of replacement mechanical contactors.

Mechanical are fine and if there are means to rapidly replace the failed one with a new one then they are far safer than any solid state one.

Plug in contactors or even a change over switch between two are ways to keep production running with minimum down time.

 

[ericmark]

Hind sight I would agree. Easy with hind sight.

However it was to show how with more semi-conductor devices the fuse is returning.

 

[bernardgreen]

It is not just with semi-conductors.

Tracks on printed circuit boards need to be protected with fast acting fuses rather than MCBs.

 

[ricicle]

Which is why semi-conductor fuses were a must in some circumstances in the industrial control environment. They have no real place in a basic circuit design complying with BS7671

 

[david911cockburn]

Sheds,

If you look at your time/current characteristics you will see that 30amp fuse wire doesn't start to be affected by load current until it reaches a value that exceeds any estimate of the current carrying capacity of a 2.5mm T&E ring final circuit.

Now compare a 32amp BS 3871 circuit breaker to a piece of 20amp fuse wire, do you see the simularity?

For quite some time I have been looking for a more scientific approach to deciding whether or not to increase the size of the fuse wire/cartridge to as much as the nominal size of the fuse carrier. Now I have found it in Table 41C, ha, ha, ha.

A 13amp plug can contain fuses of different sizes up to 13amps, why can't you see the same principle regarding fuse carriers?
It seems fairly obvious really!

 

[Spark123]

There's only one thing obvious really

 

[Spark123]

The big problem is we all I hope understand type B, C, and D MCB's but let through current of the fuse is another story. There are so many different types of fuse that most of us if designing need to look up the parameters every time.

It is the let through energy which is the issue, as MCBs are mechanical they take time to disconnect hence the energy let through where the prospective fault current is high is substantially more than a fuse.