Strange Fault

[thebassman]

Hi All

I was called to a farm to a fault in there pumping system for there water supply, this consists of a pump some 300 metres away from the farm the pump is controlled via a contactor, the coil is controlled by the float switch at farm storage tank. The fault is when float switches call for water contactor operates but when float switch opens the contactor stays in, we replaced contactor so this is a new contactor. with float switch open i measured a volatge of 120v on switch wire from float switch. So i tested the cable which is a 2 core swa and got insulation of greater than 100mOhms between core and to earth. So as you see there seems to be no leakage in the cable i also connected cable to 240v and did loop impedance test this was high at 7.37ohms, i would have thought that if there was any leakage between core appplying 230 would have caused mcb to trip. I have also disconnected float which and just linked out cable end and the same thing happen i have ran out of ideas for this one spent a a whole day at this tramping down through a field about 20 times in the rain.

Any thoughts on what could be problem i am convinced its the cable its been in 23 years and is one continuous lenght no joints in it

Danny

 

[telectrix]

have you IR tested between the switching pair?

 

[malcolmsanford]

Hi All

I was called to a farm to a fault in there pumping system for there water supply, this consists of a pump some 300 metres away from the farm the pump is controlled via a contactor, the coil is controlled by the float switch at farm storage tank. The fault is when float switches call for water contactor operates but when float switch opens the contactor stays in, we replaced contactor so this is a new contactor.This is the fault I assume. As this just developed or as it never worked.

with float switch open i measured a volatge of 120v Should there be 120v on this, or 230v or no volts? on switch wire from float switch.

So i tested the cable which is a 2 core swa and got insulation of greater than 100mOhms between core and to earth.So no problem on the cable so as you see there seems to be no leakage in the cable


i also connected cable to 240v and did loop impedance test this was high at 7.37ohms A Zs test I assume. The high reading will obviously be due to the length of the cable run, is this suitable for the protection device. ,

i would have thought that if there was any leakage between core appplying 230 would have caused mcb to trip.Not if the Zs is to high for the disconnection time of the protective device.

I have also disconnected float which and just linked out cable end and the same thing happen i have ran out of ideas for this one spent a a whole day at this tramping down through a field about 20 times in the rain.

Any thoughts on what could be problem i am convinced its the cable its been in 23 years and is one continuous lenght no joints in it

Danny

That is a long run, and was wondering when you say that your getting 120volt is that at the contactor coil, and if it is what is the coil rating ??

I would be perhaps going along lines of a volt drop problem on this.

 

[johnboy6083]

Float switches are known to be unreliable over long periods of time. I would install a liquid level relay in the tank, and level probes. It is easy to control and wire, and offers versatility with time delay ect. Have you tried linking out the float switch with a manual one to test the pump? If it works fine, then you know what the problem is.

 

[thebassman]

Hi All

Thanks for replies to add to my original thread, the coil voltage is 230 ac and the coil will stay in when float switch opens, as i stated i have completely removed float switch and with the red core connected to 230v and black normally 0 volts when switch is open, i get 120 volts to neutral or earth on the switch wire with the coil disconnected, all i can think of is that somewhere alone the cable there must be leakage between red and black cores, This pumping system has worked with no problems for the best part of 20 years, the old contactor had burned contacts and i did notice a loud buss from contactor when float switch opens i put this down to voltage being too low to hold coil in fully, the reson the old contactor packed up.

Danny

Thanks for input

 

[telectrix]

see post 2. leakage between switching conductors.

 

[thebassman]

Yes i have done Ir between switching pairs and greater than 100mOhms, this has got me pulling my hair out.somewhere along the cable 230v on the red core is leaking to the black core but is not showing up on tests on cable, when you remove supply from red core the 120v is not present on black, as i have said i am sure its the cable insulation thats at fault somewhere. The farmer does not want to replace cable due to high price of cable but i don see any other way, unless there is some form of wireless control that would work over 300metres


Thanks for replies

Danny