Intermittent tripping over Merlin Gerin Masterpact circuit breaker

[Brant]

I have an MG masterpact 2000 amp 415 volt 50 Hz 3 pole circuit breaker fitted with a STR 58U trip unit. The unit was carrying about 1000-1200 amps at the time of the trip?
It is a stand-alone unit with no external trip circuits.
It has tripped on several occasions with no apparent cause.
The unit is reset & re-closed with no problems.
I have a theory the trip unit has an earth fault setting which is a T type it doesn’t use a true earth source current transformer, it calculates the vector sum of the phase & neutral current. My thoughts are that if there is a disturbance in the power system that causes an unbalance in the Phase current could this cause the intermittent trip?
Has any one got any ideas

 

[dpc]

Doesn't the trip unit tell you if it tripped on phase or ground? I'm not familiar with this particular trip unit, but normally, there are lights or targets to indicate cause of trip.

Unbalanced phase currents should not normally cause a residual type ground relay to operate since the phasors of the three currents must still add up to zero unless current is flowing somewhere other than the phase conductors.

If there is no other obvious reason for the trip unit to operate, I'd suspect a defective trip unit or perhaps some type of electrical interference.

 

[habor]

Are you sure the thermal trip adjustment was done properly? Once I had the same problem at site. We have several trips until we recognise the thermal overload setting was adjusted at only 40%.

 

[Kiribanda]

Hi Brant,

For me I need the following information to conclude something.

1)Does your trip unit is having T (Residual connection) option or W option (Source Ground return) for ground fault protection?

2)Is it STR58UT or STR58W?

3)Is your load is 3-phase 3-wire or 3-phase 4-wire?

4)Is your main neutral of the 415 V LV system is earthed only at the transformer STAR point or have got any other multiple ground points?

Appreciate if you could provide the above.

Kiribanda!

 

[ScottyUK]

Some years ago - about 1999 or 2000 I think - Groupe Schneider ran a no-cost product upgrade under the banner "Project Spring" which replaced tripping units of MasterPACT ACB's because of a component aging problem. I never did get a proper answer whether the tripping unit became over-sensitive or failed to trip, even though we have approx. 100 of these ACB's on site. I'm not sure if the STR58U was affected - I think the problem was the earlier ST series and the later STR series was ok. It may be worth checking with Schneider.


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If we learn from our mistakes,
I'm getting a great education!

 

[bigamp]

Had a very similar problem with 2500A Merlin Gerin ACBs in 97-98. These ACBs were lightly loaded and every now and then one would trip on earth fault (they would take turns doing this). Testing showed there was no earth fault and primary injection testing confirmed that for each ACB the earth fault CT was connected correctly. You must use the special Merlin Gerin CT intended for the application and it's very easy to connect it the wrong way around!

The STR units are supposed to function OK without external control supply (control power being provided internally via the CTs) with the only restriction being that the trip settings must be in excess of stated minimums. Ours were so initially we did not provide aux power. We found that once we connected 24V DC aux power to the STRs, they worked fine with no more phantom trips. A strange one. Possibly our load current was a little on the low side resulting in inadequate control power for the internals of the STR so they did not function properly???

 

[PowerfulStuff]

Brant,
The STR58 protection units with batteries have a button at the bottom left hand corner. If you press this once power is returned it will illuminate the fault LED for the last trip.

I have also come across the fault mentioned by BigAmp. A number of our processes operate normally at 60-70%(so no problems) but will trip intermittently during maintenance periods. This was due to failing 24V supplies as they had been installed as recommended. I would recommend that they be installed in all cases to avoid the inconvenience of nuisance tripping.

Regards,
Martin