Thermal overload relay - Back up or required? 5

[gokulkrish2]

Hi everyone,

This may be a dumb question but i have this doubt and want to clear it.

All the motors in my site have a Cuttler hammer HMCP breaker feeding in series with a motor protection relay (like E3+ or 777) to the motor.

Now my question is this

Isnt the HMCP breaker capable of protecting the motor against the overload? I know those breaker have time current characteristics. So why cant we use them

Is a thermal overload relay required or its a backup?

Please enlighten me...

 

[oldfieldguy]

Nope, nope, nope...

The motor circuit protector, in your case an HMCP, is there to detect and interrupt FAULT current, as in shorted windings or feeders or contactor. It typically is a magnetic-only instantaneous trip device set to trip at some amount of current above motor starting inrush. At current levels below this, it will typically NOT operate. It has no thermal characteristic. It has sufficient interrupting rating to perform this function.

The motor overload relay is in the circuit to detect overloads, usually due to mechanical causes, and will open the control circuit to cause the motor contactor to open, disconnecting the motor from its power source. The contactor does not have the interrupting capacity to handle fault current and the motor overload relay does not have an instantaneous function.

In the event of a downstream fault, the motor circuit protector operates instantaneously before the time overcurrent function of the overload device has a chance to operate.

old field guy

 

[LionelHutz]

You can also add that UL only allows a HMCP breaker to be used with a overload where the breaker and overload were tested as a combination. You can't just stick any overload after a HMCP if it's to be a UL package.

 

[gokulkrish2]

Thank you oldfieldguy and LionelHutz for your replies.

Glad i learned something.

But i do have another question.

If the HMCP is set to operate just above the motor inrush then what is the instantaneous setting is used for in these breakers?

Lets take an example of a 100A HMCP breaker. So this breaker will trip to any value above 100A right? Now i can use any motor who's FLA is less than 100A (including the SF) with this breaker. Now if the motor inrush is 600A we would set the instantenous value to E which is 700A and so during starting the necesarry time delay is provided to start the motor....

Do you mean to say that the time current characteristics is used for faults with an impedance and have low value?

Please elaborate.

Again thanks for the infomation.

 

[jraef]

No, you are misinterpreting. A 100A HMCP is rated for 100A max continuous current carrying capacity, the 100A has nothing to do with its protection scheme. The magnetic "Instantaneous Trips" are what is set just above the motor inrush current to protect the circuit from shorts.

Think of it this way; the MCP is only protecting conductors from fast rising short circuit currents. The OL relay is protecting everything from long-term over load conditions. That is why they can only be used together in a tested listed combination; someone must insure that the amount of current that gets past the MCP will not damage or disable the OLR.

Expanding on your original question; Even if you had a "thermal-magnetic" (TM) circuit breaker, that cannot generally be used as the sole protection of the motor. The rules for the current tolerance for motor winding protection are too strict for a TM CB to be used because they only come in standard sizes. OL relays provide that closer level of protection necessary to prevent fires.


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[waross]

And the inverse time curve of the breaker will not allow the motor to start if the breaker is sized to protect the motor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

To make it perfectly clear (hopefully), the HMCP has no time-current trip function associated with it. If you set the trip current to 700A then the HMCP would trip instantaneously if the current goes above 700A but never trip below 700A.

The 100A HMCP set to 700A won't trip between 100A and 700A of current, even though it's rated for only 100A. So, the overload must do the tripping to protect both the motor and breaker from overloads. I would believe the overload having to protect the breaker in a certain current range is the main reason UL requires combination tested components.

Now, using a standard thermal magnetic breaker it could be possible to use a breaker that would satisfy both the overload and protection requirements but then you might not be able to start the motor. It really depends on the application.

In the end, the breaker is installed for short circuit protection and the overload is installed for motor overload protection. This is why it's important to not reset the breaker - the breaker should only trip on a short circuit and if it has tripped there is something seriously wrong.

 

[gokulkrish2]

Thank you so much LionelHutz, Jraef and everyone. I understood it now. Thanks again.

Kudos to eng tips

 

[jraef]

LionelHutz
Excellent expansion on the issue, I had never thought about the connection between the TOL and the MCP as it relates to the TOL protecting the breaker from over current. Seems so obvious now...


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[edison123]

Lionel - A perfectly clear explanation.

Muthu

http://www.edison.co.in