medium voltage cable overcurrent protection

[bikenut44]

I am curious why NEC article 240-101 which states the long time delay overcurrent protection on cables >600v can be set at 6x the ampacity is at odds with the ampacity tables. What is the thought process between the two.

 

[davidbeach]

Generally overload protection of MV (and higher) systems is not a function of the protective devices but is instead a function of system design and operation. The protective devices are there to protect against faults. When you get into the utility system, there are requirements that prohibit tripping on overload conditions.

 

[musaUKPN]

I am not familiar with NEC requirements; however being working at utility I know that two alarms are usually set called high alarm and very high alarm for the MV cable rating. The high & very high limits can be slightly higher than the cable rating (we call this higher rating as cyclic rating). These alarm warn the control engineers that cable is running at full rated value or even above. The control engineers then take necessary measures to reduce the loading. The cable overloading for high alarm is acceptable for longer periods but for very high alarm; it is limited to max of up to 4 hours. Sometimes if load cannot be reduced; load shedding or connection of generators can be employed.
regards,

musa UKPN

 

[bikenut44]

So an Engineer could design a underground duct bank system with the complicated Neher - McGrath calculation but then set the overcurrent protection at 3x and have to rely on the customer monitoring and not to overload it over its usefull lifespan? To me that seems like asking for trouble somewhere down the road.

 

[burnt2x]

What David said is true. Some systems are designed to take the punishment no matter what. Others are designed to clear-out quickly to prevent damage to expensive equipment. An engineer took oath to use whatever he has at his disposal to the satisfaction of the people he serves. That peoples' need could be continuity of electric service, especially on life support systems. Hence you design alarm-only systems, not auto-isolate systems. IMHO, that's no trouble waiting to happen. That's a design option.

 

[davidbeach]

If you're worried about your client abusing the conductors then set the pickup closer to - or even at - the ampacity of the conductor. The code establishes a maximum setting, says nothing about the minimum setting permissible.

 

[musaUKPN]

The decision regarding what should be an overload limit (for tripping)is open choice and largely depend upon the downstream connected load and its characteristics in addition to the cable ampacity. A low settings would also have its drawbacks like nuisance tripping during momentry faults; load transfers etc!
If the cable circuit you are protecting is feeding a single customer; you may go for lower overload settings if you wish.

Can I ask are these settings to be adapted at utility breaker or breaker at or near the customer's site?

Also consider the period of the overload. If this is of short duration < 30 min and its occurance is rare/ occasional; the impact on cable life can be ignored. The cable ampacity at design stage should be decided keeping in mind the customer's load requirements plus any future growth. If we strict to these golden rules; the cables wouldn't be subject to overload any way and we could a higher overload settings can be used.

musa UKPN

 

[bikenut44]

I typically have set relays at or just above the cable ampacity/ or below the cable damage curve. It just seems odd that you can go into meticulous detail in fine tuning the cable ampacity (ie Nehr MCgrath calculations, temperature correction factors, circulating current calculations) and in the end set it at 600% of that rating.