Hello,
Is there any requirement in the Code to provide 173% insulation level for 3 phase, 3 wire feeder? If there is a single-phase ground fault on this ungrounded delta system, the phase to neutral voltage would for the two upfaulted conductors would increase by factor of 1.73. If ground fault protection is not provided and the feeder is not disconnected from service in a timely manner, the system would be subjected to prolonged overvoltage condition which would likely damage the insulation and has the potential to become a L-L fault. Besides a good practice and requirement in the Code to provide conductor protection, are there any specific callouts on this in the code book?
Thank you,
EE
No in general. For several reasons.
First it only exists that way at the fault and it’s short term. Don’t forget insulation is tested at 200% of voltage class plus 1 kV. So a 600 V rated cable is tested at 2200 V. A 5 kV cable is tested at 11 kV. 300 V insulation if you can find it is tested at 1600 V. So even though it would be illegal for ungrounded delta it can still work.
Second at low voltage (under 1 kV) for delta or resistance grounded you must run line-line rated cable but on solidly grounded wye lower voltage is allowed. So theoretically that’s 600 and 300 V cables for most installations. But the cost difference with 300 V cable is so small that it is usually more expensive due to lack of demand. So most installations are 600 V cable for common voltages.
In medium voltage shielded cables are where insulation level applies. Cable manufacturers only make shielded cable for 5 kV and up. This ignores “temporary” hookup wire and motor lead wire for instance that are not Listed for NEC purposes, only as components of assemblies. The thing is that a 5 kV shielded cable actually only sees 2.4 kV line to ground so the manufacturers “cheat” knowing this fact. This is fine with a solidly grounded wye. So this is 100% insulation level. But with delta or resistance grounding you have to add extra. The insulation stress is radial so you don’t actually need 173%. Only 133% is needed. There’s a chart that says that if the fault can remain indefinitely use 133%. Intermediate levels can be used if you clear the fault quickly. But again we hit a demand issue so in practice really only the 100% and 133% levels are used.
Getting back to assemblies inside panels you see crazy stuff. The most popular unshielded MV cable for years was Exlar brand although there are competitors. You can get voltage stress and tracking damage if Exlar cable gets too close to another phase or a grounded metal surface. So extensive use of insulators and GP3 fiberglass (Glastic brand) and lots of tie wraps are used. And since air is insulation you can mix the two. So since anything above 15 kV is made to order often you see 15 kV cable on 25 KV or 35 KV insulators in 25 or 35 KV class switchgear. It’s not a big deal if you know what’s going on and take steps to ensure cables don’t touch each other or grounded surfaces. You have to do it anyway. In practice what I usually see is the air gaps are equal to worst case phase to phase distances and the cable may as well
Be bare so the insulation just makes it look pretty. This is at 15 kV and up. At 5 and 8 KV quite often anything goes. But after ten years or so it becomes very obvious who knows what they are doing and who doesn’t. This applies to both installers and manufacturers. Some of the biggest names (GE) are in my example photos.
Finally let’s be clear about why you do NOT want to use ungrounded deltas. You cannot insulate to a high enough level. You can insulate at 1000% and it’s not enough. Ungrounded delta just shouldn’t be used except at high voltages (69 kv+) where arcing faults are rare.
Instead for 250-8 kV use a high resistance ground. It is the cheapest ground to install and maintain. It controls transients and can still maintain power during faults like ungrounded, plus it is easy to detect and remove ground faults unlike ungrounded.,above 10 kV the system charging current gets excessive. In that case a low resistance (400 A) grounding system has most of the advantages of high resistance grounding but a smaller resistor. The resistor is usually 10 seconds rated so we must trip quickly.
