High Voltage Cable Overload Protection

[pace]

Why, with LV cables are you required to provide both overload and short circuit protection yet with HV cable only short circuit protection is required?
Why not set the overcurrent protection to provide overload as well?

 

[redtrumpet]

Your post doesn't make sense to me. No matter whether a cable is LV or HV, it has a certain maximum continuous temperature at which it can operate. That temperature will correspond to a certain ampacity. The cable should be protected from long term overloads above its rated ampacity.

Also. most if not all electrical codes are crystal clear on this subject. For example, Canadian Electrical Code Section 36, High-Voltage Installations, Rule 36-204 (1) "each consumer's service, operating unit of apparatus, feeder, and branch circuit shall be provided with overcurrent protection having adequate rating and interrupting capacity in all ungrounded conductors". Overcurrent device is defined in Section 0 as "any device capable of automatically opening an electric circuit, under both predetermined overload and short-circuit conditions...".

Are you thinking of a utility situation where the utility does not have to follow the code? However, you should still follow good engineering practise, which is to provide both overload and short-circuit protection.

 

[joan271273]

When you are working with MV feeders to another substation in a Industrial or Distribution enviroment for example you would in a simple way:

1.- Design Cable for maximum expected load and certain future growth.
2.- Cable Ampacity determination will require some modelling which will indicate allowable contnous ampcity under "X" amount of load and also the possible emergecy overloading capability.
3.- I will have a dedicated 50/51 Relay protecting this feeder which will be set for phase units at 1.25 times the maximum load for the initial deployment.

When you set this relay you are providing not only SC protection but also overload protection by the intrinsic T vs C characteristic of the relay in conjunction with that multiplier giving over the calculated amapcity value.

I hope this helps