Interesting problem I faced on work

[mttee2]

I am not sure whether this is the right section. Admin, please redirect this topic if needed.

  One month ago, one of the machine I am working with breakdown. I am quite new to this field and unsure what lies ahead for me that day. When I reached the site, I saw the mains are connected to the room's distribution panel and the distribution panel connects to the machine and room's lighting. The machine's electrical system consist of a switchboard, a 7.5 HP electric motor, an electric water pump, 5 limit switches, and 7 hydraulic solenoid valves. In the switchboard are relays, timers, and MCBs. Apparently the distribution panel's MCB had tripped.
  I turned off the switchboard MCB and turned on the distribution panel's MCB to check whether the trip was cause by the machine. The distribution panel's MCB tripped immediately. Hence I believe the room lighting system was the fault. I argued with site technician and he disconnected one of the wires in the distribution panel to my machine and the distribution panel's MCB can be turned on. I was shock and wondering why the machine switchboard MCB did not disconnect the incomming supply properly then I check the switchboard MCB. It was fine, there is no continuity readings from it but the resistance readings was quite low. It is a Hyundai MCB and the open resistance was only few hundred ohms. It makes me wonder whether the MCB was a faulty one.
  I ignore the problem and work my way to isolate the short circuit component. Apparently the water pump was burned due to something hit it and rainwater enters the motor's coil from the damaged frame. Finally I turned on the machine, it works for a minute then the thermal overload relay in the switchboard was tripped. I checked the motor connection, it was correct but the machine failed to turn on more than a few seconds. I was very clueless and no one can help me at that time. Then I check the incoming supply to the switchboard and saw the voltage was
L1 to N    0.25 VAC
L1 to L2   400 VAC
L1 to L3   380 VAC
L2 to L3   390 VAC
L2 to N    399 VAC
L3 to N    379 VAC
  I was standing there thinking why the incoming supply was extremely high and our country's phase voltage only supplies 230VAC. Then I think maybe there is a short circuit from L1 to N. I check all the connection and failed to find the fault . Finally I remembered the technician previously disconnected one of the wire in the distribution panel. I was cursing him for pulling out the neutral and did not tell me what he disconnected. Now the machine is running like charm. Only need to replace the broken water pump.

I got a few question here.
1) Why the switchboard MCB failed to disconnect properly? Why the open resistance was merely few hundred ohms?
2) When the technician disconnected the neutral in the distribution panels, why the switchboard's neutral was following L1 instead of L2 or L3?

[GeekGirl]

Then I check the incoming supply to the switchboard and saw the voltage was
L1 to N    0.25 VAC
L1 to L2   400 VAC
L1 to L3   380 VAC
L2 to L3   390 VAC
L2 to N    399 VAC
L3 to N    379 VAC


I got a few question here.
1) Why the switchboard MCB failed to disconnect properly? Why the open resistance was merely few hundred ohms?
2) When the technician disconnected the neutral in the distribution panels, why the switchboard's neutral was following L1 instead of L2 or L3?

OK going by the voltages you measured, I would say that there was a load between L1 and N, While the other phases (L2,L3) had no load to N (it may have not actually been no load, it could just be that one load was drawing way more power than the others)

It is not a good practice to disconnect Neutral from a switchboard. Most electricians think that as Neutral is EARTHED that they can treat it with contempt. BUT in reality, if there is any "out of balance" (ie single phase) load, it will make the Neutral LIVE.

I have witnessed many strange problems when neutrals become open circuit. Things like incandescent globes changing brightness for no reason (not by mains fluctuation but from really dim to actually blowing after being very bright)

You did very well by measuring not just the Line to Line Voltages (eg L1-L2, L2-L3, L3-L1) but by also measuring the Line to Neutral Voltages (eg L1-N, L2-N, L3-N).  This enabled you to find the disconnected Neutral.

It is harder to fault find in a commercial environment due to currents being larger, interactions between machinery, impossible places to work (eg lack of light, temperature extremes, lack of space)

[mttee2]

Thx GeekGirl,
That explains why Neutral follows L1. I remembered the L1 was used to power the control system. Since the control system did not energized any contactor relay for the motor, L2 and L3 are disconnected by a barrier of air. Hence making L1-Neutral resistance lower than the other 2.   

Still, I can't explain the tripped on the distribution panel's MCB when the switchboard MCB was disconnected.