16mm tails with 100A fuse ???

[Wilko]

Job today to change out old fusebox and I'd noted the 16mm tails but on pulling the fuse this morning it was 100A which I wasn't expecting. Fairly recent new smart meter installed by XYZ who are also the DNO ... Shouldn't the fuse be limited to 60A in this case? Thoughts and advice appreciated

 

[soms]

@soms you obviously need to read the byb a bit more if you are only seeing confusion coming here.

The 16th Edition was the same way, lots of grey areas and areas open to individual interpretation.

Now let's play devils advocate here,

If you actually consider that your tails are say 80cm long and you have 20cm inside the CCU, you have enclosed 25% of them. So if you enclosed 25% of a run of 10mm2 T&E and the rest was clipped direct, would you factor that into your cable rating? Or would you say it's only 25% so I won't factor it in?

Yes, you are right, I should read it a bit more!

Hello Soms it is good to think outside the box and get the old grey matter going as opposed to following the sheep blindly. See #54 for further confusion.

Ah yes, reading #54 and looking at the tables got the grey matter working!

 

[marconi]

I realise this string of posts is about meter tails but it jogged my memory about the problem of 'thermal runaway' in buried cables. Here is a short interesting piece:

http://manuals.decagon.com/Application Notes/AN_Thermal Runaway.pdf

No doubt if thermal runaway is not considered there are/may be instances where this does/could happen above ground in other installations and installation methods.

To help you ponder remember:

For a steady state temperature Ohmic heating P = Thermal Heat flow Q

1. Ohmic heating P = Isquared R

2. Thermal heat flow Q = Tconductor-TAmbient/(Rt) were Rt is thermal resistance of insulation) in the units of degrees C/Watts and Q in Watts

The thermal resistance is series combination each of the layers of insulation.

For more detail look at the Neher-McGrath calculation of the ampacity of a condutor:

Understanding the Neher-McGrath Calculation and the Ampacity of Conductors - http://www.electrician2.com/ampacity.htm

Is a circular cross section the best shape for a conductor carrying a high current? If not, what might be better?

 

[Pete999]

I realise this string of posts is about meter tails but it jogged my memory about the problem of 'thermal runaway' in buried cables. Here is a short interesting piece:

http://manuals.decagon.com/Application Notes/AN_Thermal Runaway.pdf

No doubt if thermal runaway is not considered there are/may be instances where this does/could happen above ground in other installations and installation methods.

To help you ponder remember:

For a steady state temperature Ohmic heating P = Thermal Heat flow Q

1. Ohmic heating P = Isquared R

2. Thermal heat flow Q = Tconductor-TAmbient/(Rt) were Rt is thermal resistance of insulation) in the units of degrees C/Watts and Q in Watts

The thermal resistance is series combination each of the layers of insulation.

For more detail look at the Neher-McGrath calculation of the ampacity of a condutor:

Understanding the Neher-McGrath Calculation and the Ampacity of Conductors - http://www.electrician2.com/ampacity.htm

Is a circular cross section the best shape for a conductor carrying a high current? If not, what might be better?

An interesting and informative read Marconi, thanks.