Replacement Of Merlin Gerin CB's

[keithc]

We have recently had a Merlin Gerin 630A circuit breaker mechanism fail on us, so we are having to replace it. The broken one has themal/magnetic trip settings. The Thermal is x 0.8 and the magnetic is x 5. We are having to replace it with the Merlin Gerin NS630 Compact type CB with a STR 23E trip unit. This has settings of Io(0.5 to 1.0), Ir(0.8 to 1.0), & Isd(2 to 10).
How do I know what settings to put the new trip unit to?? Do I set Io to be x 0.8, and Isd to be x 5 ?

 

[mpparent]

I am not totally familiar w/ MG breakers, but firstly, you will need to know the rating plug/sensor of your new breaker and compare that w/ the old. If the rating plugs/sensors are the same, the Io would be .8. A coordination study to verify coordination w/ upstream breakers is the safest bet. It sounds like your new breaker has no instantaneous trip setting(however there may be an instantaneous over-ride), which would correspond with the magnetic setting of "5" on your old breaker.

 

[MDom]

STR 23SE trip unit has two settings in overload protection:
-coarse setting Io between 0.5 and 1
-fine adjustment between 0.8 and 1
You can choose Io = 0.8, Ir= 1 and obtain the same setting 630 x 0.8 x 1 = 504 A as you had in old TM trip unit.
There is one difference in short circuit settings between TM and STR trip units. The tripping threshhold Im in TM unit depends on rating - 630 A in your case,so you had
630 x 5 = 3150 A adjustment in short circuit protection.
(The designation Im is the same as Isd for short circuit protection -in some version you will find Isd,in some - Im) ).But in STR 23SE the short circuit threshold is a multiple of the overload protection. If you choose 5, you will obtain 504A x 5 = 2520 A instead of 3150 A. So,you have to choose 6. Don't drop the Im(Isd) adjusnment arrow in the middle position between 6 and 7,it's not correct.
B.r.