Friday, February 18, 2011

Be Careful around 3 phase

THE DANGEROUS MISCONCEPTIONS
OF 120/240 and 277/480 VOLT 3 PHASE ELECTRICAL POWER


Recently while working in an old Ford Car dealership building in Zelienople Pa., when
I came across an extremely dangerous life threatening condition caused when a so called electrician tapped into a 240 volt 3phase 3 wire circuits to feed a breaker panel in the lower service bay for receptacles and lights .The Building is supplied with 2 different electrical services
the first is a 120/240 volt single phase service for lights and plugged in items like computers ,radios etc. which has 2 hot legs which are 120 volts to neutral and 240 volts across the 2 hot legs
the 2nd is a 240 volt 3 phase 3 wire delta service which is strictly designed for 3 phase motors and heaters
there is no neutral in this service
which was for the shops air compressor and office area large heating and cooling unit.
(See Drawings) I was running my Drill to mount some new lighting when I noticed my drill speeding up and Slowing down ,after shutting off the drill and measuring the main terminals in the breaker panel
I was getting 180 volts from a line that was to only supply 120 volts.
at ths point I knew something was wrong .
I started tracing out the conduit feeding this breaker panel when I found a large shut off switch.
which the breaker panel was tapped off of.
To my horror I found the neutral wire was taped onto the steel conduit feeding the switch
it had not been run all the way back to the main breaker panel. a very dangerous condition in it self but what was even more dangerous was the fact this switch was taped off of the 3 phase service which has no neutral wire and whose 3 individual wire legs voltage are floating between 90 to 240 volts
which means I could have gotten voltage as high as 240 volts out of the 120 volt plug.
the only safe way to get 120 volts power off of a 3 phase 3 wire service is to use what is called
a 240/120 step down dry transformer with center tap . which safely lowers the 240 volts 3 phase
to 120 volts.
now there are 2 types of 3 phase services which allow you to tap directly off of the service for
120 volts these are known as 120/208 3 phase 4 wire "Y" service which any of the 3 hot legs can be taped for 120 volts and 120/240 4 wire open delta service which has its own dangers as one of the 4 wires coming in is 240 volts to neutral
you will find this type of service on rural power areas and it is often called high leg or wild phase service. in western Pa. it is found on Allegheny Power ,GPU and Penn Power services Duquesne Light does not provide it. And the high leg conductors are usually identified with a orange paint or tape. When you look at the pole were the transformers are mounted you will see 3 transformers 2 the same size and one 1/2 as big. .A well trained Electrician can spot the type of 3 phase service coming into a building by how the transformers are wired but many can not and in the case of the wild or high leg services an Electrician may never encounter one of these services in his entire life.
Which is why this type of service has caused so many injuries. When an electrician tries to tap a 120 volt device into the high leg .If you ever open up a breaker panel in a business and see that every 3rd breaker slot is not in use
this is a good indication you have high leg or wild phase service.
Another type of three phase service which can kill you dead on the spot is 277/480 volt
3 phase 4wire services
you will typically find these on very large buildings like malls, college campuses etc. where the long wire runs losing voltage are over come by the higher voltage, the lights in the building are run on the 3 -277 volts to neutral hot legs and motors are run on the 480 volts and there is often little identification to warn you of the wall switch or light is 277volts, to get 120 volts large dry type 480/120 step down transformers are used. Often times hung above the suspended ceilings or in hall closets
Many electricians have been hurt and destroyed 120 volt lights when they try attaching them to the 277 service. I recently watched an arrogant know it all Pittsburgh registered Electrician wire a bunch of 120 volt high hat lights into a 277 volt circuit in one of the few buildings in Downtown Pittsburgh which has 277/480 service even after he was warned about the 277 service
he continued his hook up saying he was registered and knew what he as doing of course my reply to him was big deal my dog is registered, after he threw the switch there was a tremendous blue arc of power and
all the lights fried and he knocked power off to half the building luckily he only mildly flash burned his hand. At voltages of 277 there is a high degree of arcing which can totally destroy components and splash hot metal embers over everything
Needless to say when it comes time to hook into a circuit one must test and make sure what type of power there hooking into but more often than not they do not and this is why
the National Assoc. of Electrical Inspectors have the safety Slogan.
WIRING IS NOT A HOBBY USE A QUALIFIED ELECTRICIAN

2 comments:

  1. I worked as an apprentice yrs ago in worcester, Mass. one job was a large factory w/ 277/480. I had to install some 277 volt ltg. The brkr for said circuit was off yet I felt troubled, and with good reason. no power on that brkr BUT there was close to 180 volts between neutral and ground. The boss was too not worried, said "You should have known that." Thanks a lot Macki electric.

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  2. Hmm this post is truly effective; you have identified the core factors in your write-up.
    Oakville Electricians

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