regarding contactor connection

[Rajan Shakti]

how is contactor overload rating differs from its contact current rating

 

[malcolmsanford]

The rating current of a contactor is the value of load the contactor can take safely. For instance a 25amp contactor can take safely a constant load of 25amp

An over load is set to the load taken by the device, normally a motor that this contactor supplies. So you 25amp contactor could be supplying a 5amp moter. The overload would be set accordingly and if the motor developed a fault as soon as the amps rose above the overload setting it would trip and dis-engage the contactor

 

[Rajan Shakti]

ok i have a supply of 63 a coming with a breaker of 80a is installed before ,what will happen if my 63a contactor gets more current as breaker is of high capacity and what should be the breaker rating

The rating current of a contactor is the value of load the contactor can take safely. For instance a 25amp contactor can take safely a constant load of 25amp

An over load is set to the load taken by the device, normally a motor that this contactor supplies. So you 25amp contactor could be supplying a 5amp moter. The overload would be set accordingly and if the motor developed a fault as soon as the amps rose above the overload setting it would trip and dis-engage the contactor

 

[malcolmsanford]

I think what your saying is that you have a 63 amp load and your protection device is 80amps.

Not sure where you are Rajan, if you in the UK then our regulations state that a motor for instance exceeding 0.37 KW must have overload protection (Reg # 552.1.2) This therefore covers motors and it's protection and the contactor protection

As for a contactor being use for another purpose, say for an heating element. You design a circuit to disconnect on a fault or an overload in a certain time. For instance that element would need to disconnect under fault conditions in 0.4seconds and at a certain amp rating. So on a fault literally 1000s of amps can flow for 0.4secs. Therefore cable, contactors, etc are designed to take this 1000s of amps for those few milliseconds, if the circuit is designed correctly.

For overload it is similar but not exactly the same. Your 80amp protection device can run happily if it's an MCB on 1.45 times it's rating for an hour, but if your load was 1.13 times it's rating for a period exceeding that hour, then it would trip

So for 45 minutes your MCB can take 116 amps quite happily and not trip. But if you exceeded that hour on that load it would eventually trip on overload.

Bottom line even if your contactor is rated a 63amps, there is always built in this kind of factor, same as cables and such

 

[Darkwood]

I assume because he's questioning the overloads link to the contactor rating the load will be limited to the overload setting and probably wont exceed the contactor rating, manufacturers usually dont design higher rated overloads to fit lower rated contactors, also to note is the overload is to be included as part of the control circuit believe it or not ive seen on a few occasions when an overload was fitted as normal without using the NC overloads contacts to drop out the circuit..... one guy apparently an on-site electrician said he thought it would stop the power coming through anyway regardless of linking it to the control circuit.... and at the time he was on a better salary than i was :shame:........ not any more though he didn't last long in that post

 

[Brightspark2]

You need your contactor rating to be greater than your OPD rating...

So your 63A contactor is no good with an 80A MCB...

Sent from my Xperia S using next doors WIFI

 

[Darkwood]

It is often the case that you will find this set-up (contactor<OPD rating) in panels where a OPD may feed multiple contactors for motor circuits as just 1 example, the OPD is there to protect the supply cables from short circuits where as the overload will limit the load been drawn and protect the motor thus a smaller rated contactor is a common occurence albiet bad practice so i would fully agree with you on that score.

 

[Rajan Shakti]

SO its better to use a mcb before my 0.37kw fan motor after 20A contactor