automatic reset of MCB

sushil.sagi · Posted: Tue Dec 12, 2006 9:02 am Post Subject: automatic reset of MCB

can anybody suggest me how to make a circuit to automatically reset the MCB after its tripping due to a fault...??

i m a student nd trying for a project on this topic

please guide me

holmslaw · Posted: Tue Dec 12, 2006 10:19 am Post Subject:

If an MCB has tripped it will not re-set until you clear the fault. You could devise a circuit but it would be extremely dangerous.
Auto re-set breakers are only found on HV overhead power supplies they generally will operate three times and then lock out. There purpose is to feed a fault such as a tree branch laying on the line in order to make it fall off or burn away.

sm1thson · Posted: Tue Dec 12, 2006 10:21 am Post Subject:

without going into the why you shouldnt do it here is a link to a commercial one: here

holmslaw · Posted: Tue Dec 12, 2006 10:39 am Post Subject:

Just looked at the link - I wouldnt have believed it. Noitce they do not confirm if they comply with BS.
Hope Maplins got good lawyers

sushil.sagi · Posted: Tue Dec 12, 2006 10:42 am Post Subject: suggest any other project

thnx holmslaw

ok u suggest me a good electrical engg. project so tht i can work on that

thnks for tht

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Moderator's note

Please be aware that this is a DIY forum.
You may do better to discuss this with your college or lecturer in the Department of Electronics.

Mod Rupert
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bernardgreen · Posted: Tue Dec 12, 2006 12:20 pm Post Subject:

holmslaw wrote:

Just looked at the link - I wouldnt have believed it. Noitce they do not confirm if they comply with BS.
Hope Maplins got good lawyers

These are very useful devices when used properly. Used together with two relays they make the basic of a remotely re-setable circuit breaker.

holmslaw · Posted: Tue Dec 12, 2006 1:15 pm Post Subject:

Quote:

These are very useful devices when used properly. Used together with two relays they make the basic of a remotely re-setable circuit breaker.

To continue your advice "and will keep re-setting until something catches fire."
How do use them properly?

Pens · Posted: Tue Dec 12, 2006 1:45 pm Post Subject:

If used correctly I cant see a problem with them either, after all, most motor thermal overlaods have an auto-rest option.

bernardgreen · Posted: Tue Dec 12, 2006 2:08 pm Post Subject:

holmslaw wrote:

Quote:

These are very useful devices when used properly. Used together with two relays they make the basic of a remotely re-setable circuit breaker.

To continue your advice "and will keep re-setting until something catches fire."
How do use them properly?

The protected equipment is fed via a contact on a relay in series with the circuit breaker. The coil of the relay is fed from the load side of the breaker via a second contact on the relay. In parallel with this contact is a push button.

To power the equipment the push button has operated. Relay energises and latches via its second contact. The button can now be released. The relay remains latched until the power is removed by the breaker opening. When the breaker re-closes the relay cannot re-energise until the push button is operated.

The push button could be replaced by a small relay operated by a bit of logic that allows for three re-energises before locking out.

sm1thson · Posted: Tue Dec 12, 2006 3:32 pm Post Subject:

i have used them on lots of electronics projects. (sometimes followed by an electronic latch, sometimes not) they're not intended to go in your CU!

holmslaw · Posted: Tue Dec 12, 2006 4:00 pm Post Subject:

Bernard, These maplin things just have to seried with the load you don't need the hold on circuit you describe. A one way switch feeding the breaker would achieve the exactly the same. And why limit re-energises to three why not thirty.
I'd be surprised if they are BS approved and would imagine that where the word "auto" is used it refers to an "automobile" rather than "automatic".
If a breaker trips the cause should be investigated and fixed before it is switghed back on.

bernardgreen · Posted: Tue Dec 12, 2006 5:49 pm Post Subject:

holmslaw wrote:

Bernard, These maplin things just have to seried with the load you don't need the hold on circuit you describe. A one way switch feeding the breaker would achieve the exactly the same. And why limit re-energises to three why not thirty.
I'd be surprised if they are BS approved and would imagine that where the word "auto" is used it refers to an "automobile" rather than "automatic".
If a breaker trips the cause should be investigated and fixed before it is switghed back on.

The circuit is a "hold OFF" circuit that prevents the protected equipment from being re-powered when the breaker self closes. It requires that the button is pushed before the equipment get power. This prevents an endless loop --on--fault--off-on--fault--off

With sensible people the button is only pressed after the fault has been found.

sm1thson · Posted: Tue Dec 12, 2006 10:09 pm Post Subject:

holmslaw wrote:

would imagine that where the word "auto" is used it refers to an "automobile" rather than "automatic".

they are auto reset. basicaly they have a bimatalic strip inside so on overcurrent the strip heats and disconnects 20 secs or so later as it cools it reconects, if there is still a fault it gets hot again (and is still pretty hot from last time so the disconnect time is much quicker) and disconnects and repeat. -obviously this would be dangerous in some situations and hence bernards relay circuit.