Miniature Circuit Breakers

[danilo]

Gentlemen,

Good day!

just want to know what are the criteria in sizing up overcurrent protection for 100va/415v/110v control transformer using miniature circuit breaker.This is to be installed at low voltage side of kiosk substation with 11kv incomer.

regards,
Danilo

 

[Jraef]

This forum is, in general, specific to the US National Electric Code. Your task would likely require knowledge of the governing codes in the region where the installation is.

 

[danilo]

This forum is, in general, specific to the US National Electric Code. Your task would likely require knowledge of the governing codes in the region where the installation is.

Thanks for your reply..I just assumed that my question could be answered in general form as well..I could have cited an example based on ANSI standard in this case. All I wanted to know is the criteria which in my opinion maybe similar regardless of governing codes and standards used in any specific region.

Anyway, to be specific with my question,should the OCPD of 100va control transformer technically be based just on its rated current?is inrush current a big factor?taking my example as it is,the rated current is more or less 0.2A,is it safe and practical to install a 1A CB in this case?

thanks

danilo

 

[jim dungar]

Based on your size transformer, it is common to use 175% of the primary amps, although it is not unusual to see 300% used, as long as there is protection on the secondary.

 

[Jraef]

If you are using IEC MCBs, they come with different "trip curves" having mostly to do with their Instantaneous trip current characteristics. Generally, most manufacturers use what are referred to as "A, B, C, D" trip curves, whith D being the one designed for use on transformer primaries.

But your inclusion of the fact that you have an 11kV incomer is what bothered me. MCBs are generally NOT considered for use where there is a high available fault current, such as would likely be the case if you are coming right off of an 11kV transformer primary. MCBs are not typically capable of more than 10kAIC. You would want to use an MCCB because they typically have Interrupting Capacities of 30KA and above. However this is where your local code rules will come into play as well. You could NEVER do this in the US, but you may be able to in some other country with looser rules.

 

[danilo]

If you are using IEC MCBs, they come with different "trip curves" having mostly to do with their Instantaneous trip current characteristics. Generally, most manufacturers use what are referred to as "A, B, C, D" trip curves, whith D being the one designed for use on transformer primaries.

But your inclusion of the fact that you have an 11kV incomer is what bothered me. MCBs are generally NOT considered for use where there is a high available fault current, such as would likely be the case if you are coming right off of an 11kV transformer primary. MCBs are not typically capable of more than 10kAIC. You would want to use an MCCB because they typically have Interrupting Capacities of 30KA and above. However this is where your local code rules will come into play as well. You could NEVER do this in the US, but you may be able to in some other country with looser rules.

Thanks ..what is the typical va rating of shunt trip coil for vacuum circuit breaker?

 

[templdl]

The question is a bit ambiguous.
Miniature circuit breakers?
MCB (Miniature Circuit Breaker)?rated current not more than 100 A. Trip characteristics are not normally adjustable. Thermal or thermal-magnetic operation.


Miniature Branch Circuit Protection
or
Miniature Supplemental Protection

Which is it, branch circuit protection of supplemental protection?
The NEC addresses supplementary protectors in that they are not for use for branch circuit protection.
A branch circuit protective device is UL489 where the supplemental protective device is a UL1077 device.
You must provide branch circuit protection first and foremost and then can put UL1077 supplementary protective devices anywhere you heart desires.

 

[Jraef]

The question is a bit ambiguous.
Miniature circuit breakers?
MCB (Miniature Circuit Breaker)?rated current not more than 100 A. Trip characteristics are not normally adjustable. Thermal or thermal-magnetic operation.


Miniature Branch Circuit Protection
or
Miniature Supplemental Protection

Which is it, branch circuit protection of supplemental protection?
The NEC addresses supplementary protectors in that they are not for use for branch circuit protection.
A branch circuit protective device is UL489 where the supplemental protective device is a UL1077 device.
You must provide branch circuit protection first and foremost and then can put UL1077 supplementary protective devices anywhere you heart desires.

Templdl,
MCBs are not adjustable, you buy them with different curves. All manufacturers (except one) use the same trip curve designations, with D being the one you would buy it protecting a transformer.

He is obviously not from the US, his voltages are not standard here, so UL and NEC issues are irrelevant to him.


danilo,
I don't know if there is a "typical" VA rating of a shunt trip coil on a VCB. But being that it is only energized for a second at most, from a standpoint of selecting an MCB all you would be concerned with is the inrush current, which even if you knew a "typical" VA value would be vastly different from one coil to another. The only safe bet would be to get the specific data from the VCB manufacturer. If you don't know who it is (for instance if it is a project out for tender and you don't know who will win), then I would suggest looking at all of the major players and selecting the one with the highest inrush value.

 

[templdl]

Jraef,
In electrical application it is dangerous to assume and you can not assume that it is obvious and concludes that the assumption is a fact unless one asked for clarification. More time than not as an application engineer I have been asked more questions buy those who are looking to get the answer that that want to hear. They often do not know the importance if providing all of their application facts of they direct their question with the intention of supporting their opinion.
Also, It is very important to take the inrush of the ST coil into consideration when selecting a CPT. Should the CPT not be able to regulate when subjected to the inrush os a ST coil the voltage may drop to a point where the ST will not operate at all. As this story continues to evolve you will note that there are additional pieces to the story.
Now we have swgr and a ST. What does the swgr mfg recomment for sizing the CPT VA?
Then, is the ST the only item being supplied by the CPT? Has anybody seen a control wiring diagram? If not it may be a bit presumptuous to provide answers without knowing what the control scheme consists of an any which may lead to providing Danilo with incorrect information.
I is importortant to apply tha correct OCPD in the right places in the circuit. Is it a branch circuit protective device of a supplimentary protective device?
Regardless to where in the world that you are an excellent place to start is NEC and art 450 for transformers Table 450.3(B) Maximum Rating or Setting of Overcurrent Protection for Transformers 600 Volts and Less (as a Percentage of Transformer-Rated Current). This is what the NEC says about transformer protection.

Also, an OCPD placed only on the primary side to protect the transformer from an overload the OCPD must be coordinated with the pri FLA of the transformer. Small transformers have an extremely high inrush and that's whey a UL1077 supplimentary protector with a 'D' curve may be the best because of its instantaneous If the OCPD is to protect the upstream distribution system should the transformer fail taking the transformer off line then the pri OCPD is viewed in a different light. Personally I prefer to place the transformer OCPD on the secondary side using either a fuse or a thermal magnetic breaker and a pri OCPD to take the transformer off line would the transformer fail because of a shorted winding.

Yes, a UL 1077 OCPD with a 'D' curve would be the best choice for a pri OCPD id sized properly. The only diffenc fron the A, B, Anb C is that the magnetic element is calibrated higher to over ride the magnetizing inrush current of the transformer.
"Miniature Circuit Breakers"
A specific type of circuit breaker, used to switch and protect the lowest common distribution voltage in an electrical system. Generally used in a loadcenter, panelboard, or similar device. Miniature circuit breakers are available with a wide selection of accessories, including shunt trips, and can be custom modified to meet special application requirements.
These are UL489 devices.
"Supplementary Protectors"
Supplementary Protectors are ideal for providing protection in a multitude of applications. A current limiting design provides fast short circuit interruption that reduces the let-through energy which can damage the circuit.
Supplementary Protectors are ideal for providing protection in a multitude of applications, including:
Motor Control Circuits
Control Power Transformers
Relays
Contactor Coils
PLC I/O points
Lighting Circuits
Current Limiting design provides fast short circuit interruption that reduces the let-through energy which can damage the circuit. Thermal Magnetic Overcurrent Protection provides:
Three levels, categorized by B, C and D curves
Standards and Certifications
These are UL recognized under UL 1077
CSA 22.2
IEC 898
IEC 60947-

Incidentally also include with supplementary protectors are Hydraulic-Magnetic Breakers

Again, supplementary protectors are UL1077 devices.
I would like to emphasize that this IEC breaker is recognized by UL under UL 1077.
The UL1077 devices are used where branch protection is required (for example UL 489 MCCB) that is already provided or not required.

Regardless of where you are and what codes may apply if always makes since to provide branch circuit protection first and then use supplemental protectors where it may sense to provide additional coordination.

 

[Jraef]

Jraef,
...
Again, supplementary protectors are UL1077 devices.
I would like to emphasize that this IEC breaker is recognized by UL under UL 1077.
The UL1077 devices are used where branch protection is required (for example UL 489 MCCB) that is already provided or not required.

Regardless of where you are and what codes may apply if always makes since to provide branch circuit protection first and then use supplemental protectors where it may sense to provide additional coordination.

No disrespect, but you seem to miss my point. Your information s not incorrect, but the issue of UL1077 vs UL489 is relevant only to someone USING the component in the US, or maybe sending equipment to the US. Outside of the US, they do in fact use MCBs in the same way we use "branch circuit protection". In fact the concept of "supplementary protection" or "branch circuit protection" as being different, and supplementary being something less robust, is a construct of the UL-centric system we use here.

His primary voltage was stated t be 415V. I know of no place in North America where 415V is a standard voltage. It is, however, a very common voltage standard in many other countries in the world. So I think it was a reasonable assumption on my part that the entire argument over UL1077 or 489 is moot to the OP. He was not asking about that anyway, he wanted to know how to select the right rating. We still have no idea what country, or codes, he is subject to, so as I posted in the very beginning, this may not be the appropriate forum for his question. The tangent this thread has taken is, IMHO proof of that.

But I tried anyway. I surrender...

 

[dbuckley]

This is one of those application where a fuse in the primary is worth a home depot's full of breakers. Breaking a gazillion amps is not the job for a mcb.

Save a breaker for the secondary.