RCD protection of a 3 phase system

[1000V]

Hi guys, I have tried searching without much luck. This, I'm sure I should know this but I'm getting a bit swamped.

I have on site a breaker feeding a 3 phase dis board. The breaker obviously has a 3 phase (100A) overload unit but also has an RCD. The RCD is fed from the secondary side of a CT, all three phases and neutral pass through the aperture of the CT. The CT ratio is 1000/70 which I find quite strange.

The RCD is set to 5 amps and we have been experiencing some trips at the feed over the past 3 days. The RCD is a starkstrom 250.

I have measured the current on each phase and found them to be: Red 40A yellow 35A and blue 25A. There is 25A on the neutral. There are RCBO's on all of the circuits on the dis board rated at 30mA and none have tripped. We replaced the RCD but still are getting trips.

I've calculated what the neutral current should be and come up with 13.2 Amps. I used the formula method rather than the vector diagram method. I squared all the phase currents, multiplied each phase current by the next and subtracted the results. I then took the sqrt of the result.

I have two questions:

Firstly; Why is my neutral current much higher than what the neutral current should be on an unbalanced 3 phase system, or rather my calculated result, and could this be the reason for the trips?

Secondly; Could somebody help me out with the method of calculating the current on the secondary side of the CT? I would be fine with a single phase system, but when it comes the 3 phase currents and the interposing neutral current I'm getting a bit lost.

I am very sorry if this question has been asked before or it is something rather trivial.

Many thanks, T

 

[waross]

Tripping; Your neutral may be grounded somewhere downstream.
High neutral current: It may be that you have harmonic currents generated by line to neutral connected equipment.
Calculating the expected neutral current. Do you have any idea what your current phase angles are? Currents with differing power factor will be at different angles. Line to neutral load currents will be at differing angles than line to line load currents.
That's for 60 Hz. Third harmonic currents generated by line to neutral loads will add on the neutral rather than canceling.
For your problem:
1> Check for downstream grounds on the neutral.
2> Check (or) replace the CT.
Suggestion. Place a second CT around the phase conductors and the neutral, next to the existing CT. If it shows zero current then change your CT.
If the second CT shows a current that agrees with the existing CT, check for a grounded neutral.
Note: there ust be no connections of any type at all to the conductors between the existing CT and the test CT. If you eventually come back with a statement such as, "Well, I couldn't get the CT close to the other one but it fit good the other side of the ground jumper." you will have shot yourself in the foot before you have started. .

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[1000V]

Thanks bill I will let you know how i get on!