3 phase

[varnerelt]

I have a problem with a 3 phase knife switch the line side of line a is hotter than all hell while line b and c are cool they all pull the same amps yet the load side is also cool i replaced the wire of line A all wires pull 90 amps max they are all 3/0 copper i can't not find why 8 inches of the line side is hot coming into the knife switch and the fuse is hot i mean hot enough to almost burn oneself but the load side is not if anyone has any ideas please get back to me...at one point before i took the job the wire got so hot it burnt off the wire right at the enrty of the lug so i replaced the lug at the same time as the wire..now is it possible that if the blade was dirty and pitted this would have the same effect?

[ May 23, 2003, 03:47 PM: Message edited by: varnerelt ]

 

[charlie tuna]

Re: 3 phase

being in the infrared business for over fourteen years i would suspect bent knife blade on "a" phase. the heat produced by this poor connection will rise up into the line side wire. i'm assuming you've already checked the line side wire torque to be correct. chances are the switch rating is close to the current carried across the blade? if it's 90 amps and the rating is 100 amps, any small problem will cause the poor connection to quickly reach a failure condition. the bad blade may have been caused by the installer overtorquing the line side wire terminal and bending the blade. once this happens the switch is shot! another problem area might be the lug connection to the switch - we have found some loose right from the factory. you have to remove the switch's guts to hold the backside of the bolt....

 

[dereckbc]

Re: 3 phase

I agree with Charkie, you have too much connection resistance either in the knife blade or terminal connections.

 

[brian john]

Re: 3 phase

A simple test to isolate the source of the heat in lieu of performing an IR scan would be to perform a FOP or fall of potential test across the poles of the FSS.

Read from line to load of each pole breaking the switch into sections line to knife blade, knife blade to top of fuse, top of fuse to bottom of fuse, ect. You will need a multi-meter with a millivolt scale. A typical reading at this level of current would be between 20 to 80 millivolt. The real results are comparative readings between the phases. A phase line conductor to A phase knife blade compared to B phase line conductor to B phase knife blade