Overcurrent protection

[DWCOCKBURN]

How do we protect an electrical circuit from 'a small overload of a long duration'?

 

[westward10]

Specify "long" and "small".

 

[DWCOCKBURN]

Why?

 

[Brother Blue]

If the duration is sufficient for the over current protection to operate then that provides the protection.

We are required to design circuits to minimise the risk of small overloads affecting the circuit but also the current ratings given for cables etc have a tolerance built in to them to cope with small overloads that don't cause protective devices to operate.

If a more fine/precise protection than is afforded by standard over current protective devices is required for a particular application then additional protective devices can be installed, such as used in motor controls.

 

[DWCOCKBURN]

I can't argue with that.

Would you agree that the easiest way to achieve this would be; to make sure that the current carrying capacity of the smallest conductor within the circuit exceeds the I2 value of the chosen overcurrent protective device?

 

[Brother Blue]

I don't have the time right now to give this much thought, I'll come back to it later.

It would be good if you could just tell us whatever it is you are leading up to with this rather than going through a drawn out process of baby-stepping towards it.

I am of course assuming, based on your previous contributions, that you are going to share with us an idea which contradicts current thinking and installation practices?

 

[DWCOCKBURN]

Give it some thought.
Perhaps somebody else might like to contribute whilst you are thinking.

 

[Brother Blue]

I have given it some thought and as it stands I am happy to accept the current position of using the 1.45 factor for circuit breakers and cartridge fuses.
As it stands I see no reason to go against this convention.

Perhaps you could offer up a well reasoned explanation as to why this may not be suitable?

 

[DWCOCKBURN]

Can a ring final circuit be subjected to 'a large overload of a short duration'?

 

[Brother Blue]

Yes, of course it can, and the larger that overload is the shorter it's duration will be as the overload protection operates.

 

[newfutile]

433.1 PROTECTION AGAINST OVERLOAD CURRENT
Coordination between conductor and overload protective device
Every circuit shall be designed so that a small overload of long duration is unlikely to occur.

 

[DWCOCKBURN]

Yes, of course it can, and the larger that overload is the shorter it's duration will be as the overload protection operates.

How?

 

[DWCOCKBURN]

How can a ring final circuit be exposed to 'a large overload of a short duration'?

 

[Brother Blue]

How can a ring final circuit be exposed to 'a large overload of a short duration'?

Plugging in and operating an excessive number of high current appliances at the same time will create a large overload, the overload protective device will handle the short duration part.

 

[DWCOCKBURN]

You mean that you would have to switch on the entire circuit, with everything plugged in and switched on ready to go.

Does this happen often?
Or does it tend to coincide with periods of time when you have nothing useful to do?

 

[mainline]

You mean that you would have to switch on the entire circuit, with everything plugged in and switched on ready to go.

Does this happen often?
Or does it tend to coincide with periods of time when you have nothing useful to do?

You could have timed heaters plugged into the socket outlets in different rooms, all on the ring final set to come on at the same time.

I know of a few rental properties that have them, they replaced a 12kw electric central heating system that caused a fire in the one property.

 

[Brother Blue]

You mean that you would have to switch on the entire circuit, with everything plugged in and switched on ready to go.

Not necessarily, a number of heaters could be used as the load.

Does this happen often?

Not very often in my experience, I can only recall seeing a ring circuit significantly overloaded when a heating system had failed and multiple heaters were plugged in.

Or does it tend to coincide with periods of time when you have nothing useful to do?

This seems like an entirely unnecessary comment

 

[DWCOCKBURN]

The question that remains unanswered is:
Do you agree that "making sure that the current carrying capacity of the smallest conductor in the circuit exceeds the I2 value of the chosen overcurrent protective device, is the 'easiest' way of protecting an electrical circuit against 'a small overload of a long duration'?

 

[Brother Blue]

No I don't agree because I don't see how that would protect against such an overload as it would not stop such an overload from happening.
Using I2 to select a cable size would not change the In of the circuit or OCPD and so any such overload would happen exactly the same.
What it would do is reduce the temperature rise in the cable during such an overload, it would not change how anything else in the circuit is affected by the overload.

If it is necessary to protect against small overloads in a circuit then a protective device capable of offering such protection would be the answer.

 

[mainline]

I’m just going to say no

 

[DWCOCKBURN]

The evidence suggests that you are correct about the temperature being the key, but experience has demonstrated that it will be the join between the line pin of the 13amp plug and the socket outlet where the point of ignition will be found.

 

[DWCOCKBURN]

If you choose a 32amp BS EN circuit breaker it should therefore be matched with a 4.0mm conductor ring final circuit and earthed metal-clad socket fronts.

 

[Brother Blue]

The evidence suggests that you are correct about the temperature being the key, but experience has demonstrated that it will be the join between the line pin of the 13amp plug and the socket outlet where the point of ignition will be found.

Well you really have wandered off into the rhubarb now haven't you! You've jumped from discussing small overloads to points of ignition.

If there is ignition at a point such as you describe then it will be the result of an arc which can be protected against by an AFDD.

Changing the size of conductors will not do anything to help this new situation you have raised

 

[Brother Blue]

If you choose a 32amp BS EN circuit breaker it should therefore be matched with a 4.0mm conductor ring final circuit and earthed metal-clad socket fronts.

Could you explain this please because, do far, you haven't provided anything to support this statement.

A standard ring final circuit can have 2.5mm conductors and the material that the socket is made from is irrelevant to the issue of overloads whether they be small, medium or large.

 

[DWCOCKBURN]

Look at the photos of Grenfell.

 

[Brother Blue]

Look at the photos of Grenfell.

Why? What has that got to do with this?

As far as we know that fire was caused by a faulty appliance, not what we are discussing here

 

[DWCOCKBURN]

Take a very close look at the photo's then.

 

[Brother Blue]

Why, that won't help your point at all.

Could you please just explain your point, make a well reasoned technical post so we can actually understand your point and then discuss it.

A statement that we just need to look at some photos of a disastrous fire which appears to have no relevance to your point is not helpful and doesn't explain anything.

 

[DWCOCKBURN]

A Faulty fridge freezer, a fuse then wrapped in silver paper, the point of ignition was the join between the 13amp plug and the socket outlet, a 32amp BS EN MCB in the 'ON' position, after the fire burned the insulation from the conductors, a short circuit then operated the RCD.

 

[DWCOCKBURN]

Teenagers understand the theory of continuity, therefore electricians must understand overcurrent protection.
Unless you plan to blame the childlike mind that wrapped the fuse in silver paper.

 

[Brother Blue]

When and where has a fuse been wrapped in paper and what does that have to do with the overload protection of circuits?


You are making less and less sense.

 

[DWCOCKBURN]

I'm not an expert but as far as I know; there shouldn't be any need to take a three pin plug apart before you can X-ray the fuse.

 

[mainline]

I'm not an expert

I’ll agree with that

but as far as I know; there shouldn't be any need to take a three pin plug apart before you can X-ray the fuse.

x ray it for what ?

Some have removable fuses no need to take them apart.

 

[DWCOCKBURN]

To see whether the fuse element was still intact.

 

[mainline]

To see whether the fuse element was still intact.

It must have been open circuit otherwise why was it wrapped in foil.

I don’t think you would need an x ray machine to test for that.

 

[DWCOCKBURN]

Take a look at the photo of the X-ray.

 

[Brother Blue]

What photo of an X-ray?

If you want us to look at a picture the please post it up so we can see it.

And to answer your earlier point yes you would need to remove the fuse from a plug in order to get the best possible X-ray image.

 

[DWCOCKBURN]

They had to clean residue off of the fuse before X-raying it, then they added a photo of the X-ray into the report that was available online.
The fuse element was still intact.

 

[DWCOCKBURN]

It is Friday, I have had enough for one week.
Have a nice weekend.

 

[DWCOCKBURN]

Hang on a minute!

Does this mean your 'friends in high places' have been removing evidence from the internet?

Not to worry, I imagine the Police will have copies.