Wednesday, August 1, 2012

Grounded 3 Phase power legs a Dangerous condition which puts service techs in danger everyday

A  newly trained service technician  is called to check on an air handling unit on the roof of a plant he starts by checking to see if he has 480 volts of power coming off  each leg of the 480- 3 wire Delta supplied power from the utility company. He tests with his meter from phase A, B and C to ground A and C show 480 to ground however B shows 0 voltage he assumes the fuse is bad and makes a very bad judgement call of reaching in with his hand instead of a fuse puller to pull the fuse out with out shutting off the switch a rookie mistake made by many over the years that never should be done. As he reaches in he receives a severe shock and burn lucky he survives.
But what went wrong the phase said 0 volts it should be dead right? Wrong  This rookie along with even many experienced electricians who are not familiar with 3 phase delta supplied power  can run into this condition any time they work on one of these services do to the three phase power being supplied by the utility having on of its 3 power legs grounded accidentally some where within the distribution system. This condition exists in 230 volt 3 phase services provided by Duquesne light company in the North side of Pittsburgh  and 480 volt systems in Braddock Pa.
The correct way to test a 3 phase delta system is to make measurements from A to B - B to C and A to C   not to ground  this would have shown the rookie the power was actually working on all 3 phases.
Better yet he should have a Phase Rotation meter many Industrial Electricians and Trouble Shooters like my self carry  around $100-450 dollars these meters show you when all 3 phases are properly working and the rotation of the phases  as you can see in this photo of a Rotation tester made by BEHA now Greenlee  which I was using to test 3 phase power on an overhead crane.

In the case where I was checking a crane the grounded phase threw the maintenance man off  track and they called me in thinking they had a main phase fuse blown in the distribution gear  Bus bar system when in fact it was a small 3 1/2 amp fuse used to supply control voltage to the cranes operating pendant .
I tested and new power was good but to show that it was I brought the rotation meter in and hooked it up and showed it to the maintenance people so they would know in the future they have a grounded phase issue.

Now your saying to your self should equipment not be marked warning about this and should the utility company not be doing something about it? yes they should but again they do not because only properly trained and equipped personnel like my self should be doing this work but  we all know this is not the case and every year techs get hurt and confused by this condition I mark switches  and warn electricians and maintenance people about this all the time because 99% know nothing about this condition.  Duquesne Light  trouble shooters have spent many hours tracing and trying to find the ground to no avail . They should do a better job disseminating this to techs and electricians but they do not  so its proceed with caution which is what you are suppose to be doing in the first place and when a tech hits this strange condition and he and his supervisor can not figure it out they are suppose to call the power company who would send out a trouble shooter who will test and alert them to the issue but as usual they do not.  Its a dangerous world out there and thats why only properly trained personnel are suppose to be doing  this work and any one who proceeds with out the training shame on them.

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