Ice Storm Corner Pole With a Critical Customer Beyond It
Freezing rain has brought down a three-phase 25 kV tap. Phase C is on the ground in a hay field 40 m from the road, the corner pole at a 60 degree line angle has a broken crossarm and is leaning 15 degrees, and the outside guy, a 3/8 in strand with an 8 in anchor driven 1.2 m, is slack. Conductors are 336.4 kCM ACSR and the pole is Class 4. The recloser has tripped, reclosed twice and is now in lockout. A water treatment plant 4 km beyond the corner is out of service; at 800 kVA it is the largest single customer on the feeder. A normally open tie to Feeder 7 is 1.5 km past the plant. It is 02:00, wind is 40 km/h with ice still on the conductors, and the farmer's cattle are in the field.
1. The area control room asks whether the feeder can be re-energized to pick up the water treatment plant before the pole is repaired. What is your answer, and what must be true before the plant can be fed?
Read the worked answer
No - not with the faulted section in service. Phase C is on the ground in a field with cattle in it. Every reclose puts step and touch potential into that field and puts arc energy where a person or an animal can be. The recloser has already tried twice and locked out; that is the system telling you the fault is still there.
What I would do instead:
- Guard the downed conductor first - standby, warning devices, and police or the farmer notified about the cattle, before any switching discussion.
- Isolate the faulted section with visible opens on both sides - the sectionalizing switch upstream of the corner and the switch downstream - tag them, and ground at the work pole so the crew can work safely once the storm work starts.
- Restore the plant by backfeeding through the tie to Feeder 7, but only if all of the following are true:
- The tie and the section between the tie and the plant are proven clear of the fault (the faulted section stays isolated at both ends).
- Phase order at the tie is confirmed. If it has never been proven, phase out with a phasing set before closing; a wrong phase order will damage the plant's motors and pumps.
- Feeder 7 can carry the added 800 kVA plus its own load - check its peak, the tie switch rating and the conductor between the tie and the plant.
- Protection coordination still works with the tie closed, and the plant's fusing and the transformer fusing on the backfed section are correct.
- No crew is working and no grounds are applied anywhere in the section to be energized.
- If any of those checks fail, the plant waits and the customer's operator is told why.
2. What must be checked before the corner pole and its guy are re-used, and what would you specify instead?
Read the worked answer
Calculate the load rather than eyeballing it. At a 60 degree line angle the resultant pull per conductor is 2 x T x sin(30 degrees) = T, so three conductors put roughly 3T at the pole top before wind and ice are added. At a normal final tension near 2,000 lb per conductor that is about 6,000 lb of transverse load. A Class 4 pole (the lowest commonly used distribution class) is not rated for that with three 336.4 kCM ACSR conductors, and the 3/8 in guy with an 8 in anchor at 1.2 m depth is undersized for the guy tension that load produces and is too shallow for the soil.
That explains the symptoms: broken crossarm, 15 degrees of lean, slack guy. The structure was under-built for a 60 degree angle, and the ice storm finished it.
What I would specify:
- Replace with a Class 2 or Class 3 pole, or convert the corner to a guyed dead-end structure with two guys.
- New guy sized for the calculated load with a guy strain insulator, and a screw anchor or plate whose holding capacity in that soil exceeds the guy load with the utility's safety factor. Confirm the anchor can be installed inside the easement.
- Check the guy lead for clearance to the road and for minimum approach distance to the conductors, and check the pole setting depth and backfill.
- New crossarm and insulators, not re-used ones. Inspect the remaining poles on the tap for lean and crossarm damage before re-energizing; a storm that breaks one corner usually damages others.
3. The downed conductor shows arc pitting and broken strands. How do you decide between re-stringing, splicing and replacing, and what else must be checked after the repair?
Read the worked answer
Use the utility's conductor repair table, then apply judgment on top of it. 336.4 kCM ACSR is a 26/7 construction. Light scuffing is acceptable; a couple of broken outer strands can be dressed and reinforced with armor rods; more than the table allows, or any arc pitting, means the conductor section is finished.
Arc pitting is the deciding factor here. Pitting means the aluminium was melted and the steel core may have been affected, so the conductor has lost strength even if the remaining strands look intact. It also sat on the ground while the recloser reclosed twice, so expect further pitting and possible annealing along the span. My call would be to replace the damaged length with a compression splice or a new span section, not to re-tie over it.
After the repair, before the section is re-energized:
- Check the sag against the stringing chart for the actual temperature; a conductor that was stretched on the ground will not match the original sag and may reduce clearance.
- Inspect the next span and the ties at the adjacent poles for damage from the same fault current.
- Check insulators for arc tracking, and check the cutouts, arrestors and pole ground.
- Walk the section, remove all grounds, confirm the switch and tie positions, and if the tie was used, re-check phase rotation at the plant before handover.