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