induced voltage

[shamrami]

i have an intertrip circuit, where the distance between the down stream and the upper stream relay is around 0.5mile. the problem is that i am getting nuisance tripping due to availability of 250V AC across the uper stream relay causing it to operate. This relay operates only on a 250v when it is available accros its terminal. the control wire is going all the way from the down stream relay contact to the upper stream contact through a pipe. when this contact closes (relay opertes)it closes the circuit to supply 250V to the upper stream relay through PT 480/250 V.

Please advice what might cause this tripping during normal operation.

 

[Skogsgurra]

shamrami,

Sounds like you have guessed the reason for your nuisance tripping already. 1/2 a mile (800 m) is a long distance and it is very well possible that the capacitance of the wires play a role here. This wire represents around 50 - 150 nF of capacitance (depends on cable type and area) and that can easily pass a substantial current that trips the relay. Sometimes you even get a resonance between cable capacitance and relay coil, but that usually makes the relay stay activated when the distant switch is opened. It seldom pulls the relay in.

If your relay is a small one with low coil current, it is a good idea to try a small contactor instead. They are much less susceptible to this problem.

The easy way to get rid of the nuisance trips is to put some extra load parallel to the relay coil. An incandescent lamp usually works well. If you put a 15 - 40 W lamp across the coil, you sholdn't have any problems at all.

But, you have to be careful. If it is important that the circuit trips when it shall (usually very important) you should A) Check that tripping takes place also when the extra load is added and B) Make sure that a short in the lamp does not blow the fuses in the trip circuit. A small extra fuse (0.5 or 1 A) in series with the lamp is wise to have. If you select too high a melting current, it is possible that the fuse doesn't blow. The short will then load your trip circuit instead of being cleared by the fuse.

If you want to do something a little more elaborate, you can change to DC and use a DC coil and a DC rated contact in the signal path. It is more complicated and not always possible to find a DC coil with the same dimensions and function.

 

[busbar]

Roughly similar problems have been discussed at
thread237-83120
thread237-74797
thread237-36930

 

[stevenal]

I'll agree with Skogsgurra, except that to better ensure against false tripping, I'd select something more rugged than a lamp filament to drop the voltage. A heater or power resistor for example. Although the lamp is rarely fully on, it would still be susceptible to vibration or accidental breakage.

 

[busbar]

Is the 250V-secondary transformer [source] located at the local/relay-contact end of the line, or is it located at the remote/relay-coil end of the line?