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How to Adjust Old Breaker

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Nicholas Farr22/11/2020 15:02:29
2878 forum posts
1295 photos

Hi Avon, I imagine that moving the clips out, increases the gap between the bi-metallic strips and the trip bar. this will mean the bi-metallic strips have to bend slightly more before it trips the bar.

Phil Whitley, I believe you are correct, looking at my old coil contactor with the same type of trip, I can agree that there is no need to wire in the third contacts and overload.

Regards Nick.

Avon22/11/2020 15:28:42
71 forum posts
26 photos

HI Nick,

Spot on - there's a slight raised area on the clip which is moved away from the trip bar when pulled out.

Interestingly, the option of dividing the 'live' across the contacts would effectively reduce the current in each. In order to simply ensure consistency between phases they would need to be wired in series.

Regards Peter

Harry Wilkes22/11/2020 15:41:02
1134 forum posts
64 photos

As a young man I worked at a metal finishing company where this type of MEM was used and in the case of the one's used on single phase as pointed out earlier the load was routed through all 3 contacts and the overload fitted were of the correct current.


Nicholas Farr22/11/2020 16:12:31
2878 forum posts
1295 photos

Hi Avon, if you were to use all three, the only way you can do that is by wiring all three in series or two in series on live or neutral and one on on the other, the current would then be the same in all three overloads. If you were to wire all three in parallel on the live, they would still all carry the same current, but the trip current overall would be higher which you don't want. When used in three phase only one of them would have to carry more than the trip current to trip the contactor, likewise in single phase use, so don't wire two in parallel and one on it's own if you want both live and neutral to be disconnected when in stop mode.

Regards Nick.

Phil Whitley22/11/2020 19:02:37
1354 forum posts
147 photos


Nice one.


Mike Poole22/11/2020 19:25:26
3015 forum posts
71 photos

The question of whether to series the unused contact on a starter suitable for three phase is an old chestnut, if you can find the instructions for the starter some will recommend the series method and some will say it’s unnecessary. As the instructions are usually missing or unreadable I tend to series in the unused contact on the basis that it will do no harm even if it is unnecessary. A healthy motor should have a balanced live and neutral current or balanced phases and should operate the mechanical trip in a nice even way but a failing motor could have unbalanced currents in the 3 phase case or if a single phase motor was losing current to an earth fault then the return current could be different, in these cases you would still want the trip mechanism to operate so the trip mechanism should tolerate just one bimetalic operator tripping it.


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