Hi Ranger hits power line and burns

[Dadatwins]

Posted this here instead of equipment since I am looking for thought of the folks that use the equipment, if it needs to be moved that's ok.
One of our 2001 hi ranger contacted a 34kv line at the knuckle, power line arched and broke, fell to the ground, the lower boom had smoke coming out of the area near the knuckle. The boom was still operational with the engine running. The operator was able to cradle both the upper and lower boom. As the boom was cradled, the smoke started getting heavier and some oil started running out the bottom of the lower boom. The fire dept had been called upon the line contact and arrived in time to extinguish what appeared to be a fire inside the lower boom. After the fire was out and the jacks lifted 4 burn marks were on the ground where the pads were. The jacks were on the street pavement directly, no blocks/pads in between. I was not at the scene but talked to operator, he did contact only the one line no other lines involved. It is my thought that a buildup of dirt, sawdust, grease built up inside the lower boom and started to burn when the contact was made. As this debris burned it finally was hot enough to melt through the hydo lines and that oil fed the fire. This dirt buildup allowed the current free path to the ground and created the arch. The upper boom fiberglass insulation fortunatly did not allow the current to travel up to the operator. My thought is that the lower boom insulation was not effective because of the grease and dirt buildup. The boom had passed a dielectric test about 6 months ago. The inside of the boom has never been cleaned since being put into service. The truck is currently at the vendor under investigation to determine cause of accident. Does anyone have any thought about this or ideas about possible cause?

 

[Al Smith]

Line contact

A contact on 1 Phase of 34.5 kv,will be 19,900 volts,to ground.The metal parts of the hoses,as well as the metal part of the boom , the leveling and lift cables ,will carry the fault path to ground,through the out riggers.Any thing in the path,will be subjected to this fault current,thus the smoke etc.I have seen 19,900,burn a cross arm completely into. It had probabley arced through the hyraulic lines,and raised the temp.of the fluid in them,almost instantly,thus resulting in a fire.It is fortunate that the out riggers were in ground contact,thus a more direct route to ground,for the fault path,and,most important,no one was hurt.

 

[glens]

How much more than nothing happens if the stabilizers are not making a circuit to (are insulated from) ground?  So how is it really a good thing that they did in that case?

Glen

 

[Crofter]

Glen, At least with a good path to ground it burned off or tripped the conductor instead of just livening up the unit as a trap for anyone touching or trying to ditch. Wooden pads are mandatory on a lot of jobs even on concrete, but I sure wouldnt depend on the insulating qualities of some of the wet old slugs I have used.

 

[glens]

I guess I was thinking in terms of momentary contact -- a quick brush.

Glen

 

[Al Smith]

19,900,will find a path to ground,even if it arcs over the tires.What is better,is a low impedance ground,whereby the fault current,is shunted around the cab,fuel tanks etc., and also generates enough fault current to trip the oil circuit breaker,or fuses,if this be the case .You can't imagine,unless you have seen,first hand,what damage this high of voltage will do.The bucket trucks that I am familiar with [electrical work],only the upper boom section,was electricaly insulated.It's a moot point,but the safe working distace on lines up to 33kv,is 3 meters,34.5 would be 5 meters.This stuff,demands respect,you seldom get a second chance. We also had static straps on the trucks,whereby the path to ground,if there was a contact,could be more so directed.

 

[Dadatwins]

Received report from vendor about burned up boom. Truck had been dielectric tested for 46kv last year and passed. After disassembly of the boom they found burned ash and some burn tracks along the lower boom. The vendor believes that a buildup of saw dust, leaves and debris along with some grease buildup inside the boom caused the lower insulator to fail. The boom hit the wire and ignited this debris causing fire which melted hydraulic lines. While the truck is electricly rated higher than the 34kv line that was hit by the boom but the contaminated conditions were part of the cause of the accident. The trucks lower boom had not been inspected or maintained for some time. A new policy at work will be put into place to have the trucks maintained and inspected regularly. While the operator should not have touched the wire and maintained proper clearance, the boom would have been insulated had it not been contaminated with debris. We are very lucky no one was hurt by this incident. The entire boom will be replaced at a cost of 29 thousand dollars. Lot of $$$ for a preventable accident. Another message to those that run these things make sure they are maintained properly, an hour or two of maintainence a week could have prevented this accident and thank goodness no lives were lost. Be careful.

 

[Al Smith]

On the subject

Where I work{Ford Motor Co],the policy is to inspect,every day,the entire workings of the lifting equipment,prior to usage.There are written cards,with a check list ,for that purpose.At one time,I thought this was going a little overboard,but,with this,the second report of a boom failure in several weeks,on this site,I now see the point.This would be a good practice for all to follow.The old adage" a stitch in time,saves nine",has taken on a new meaning,much more serious.