I would like to know if anybody has used circuit breakers in a power distribution circuit where the upstream circuit breaker and the downstream circuit breaker ( maybe incomer to another distribution board) have different short circuit ratings. There is no fuse used anywhere in the circuit.
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Circuit Breakers with different short circuit ratings 12
- Thread starter uno
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Or have you seen a power distribution circuit where the upstream circuit breaker is from a MCC/PMCC and feeds another distribution board which has outgoing breakers having lower short circuit ratings. There is again no fuse used anywhere in the circuit.
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How far upstream are you talking? Is there any equipment in between? Upstream breakers have less impedance between them and the source, therefore will see (and must be rated for)higher SC currents.
If there is no impedance in between the two breakers,the downstream device can be series rated as per manufacturer specs and testing. This has been discussed in this forum previously...maybe try keywords "series rated"??.
If there is no impedance in between the two breakers,the downstream device can be series rated as per manufacturer specs and testing. This has been discussed in this forum previously...maybe try keywords "series rated"??.
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Thanks Pals
Should have used keyword search. It is available in :
Now I want to know if anybody has used series rating/ Cascading applications with selective discrimination in any of your industry. I have read some literature but would like to know if it has been carried out in real life situations especially in three phase power distribution schemes. I know it is being done for lighting .
Dandel
I would also like to know as to how you think it is possible. Have you seen it anywhere. I do not want to take the cable impedence or the presence of a fuse in the scenario to reduce short circuit levels.
Thanks to all
Should have used keyword search. It is available in :
Now I want to know if anybody has used series rating/ Cascading applications with selective discrimination in any of your industry. I have read some literature but would like to know if it has been carried out in real life situations especially in three phase power distribution schemes. I know it is being done for lighting .
Dandel
I would also like to know as to how you think it is possible. Have you seen it anywhere. I do not want to take the cable impedence or the presence of a fuse in the scenario to reduce short circuit levels.
Thanks to all
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uno,
If you use U/L labled molded case breakers check out what the interrupting rating really means. The U/L test method calls for about 10 feet of wire on the load side of the breaker during the test. The wire used will be the maximum size permitted for the breaker size. On the smaller breakers the 10 feet is sufficient to cut the fault from about 35kA down to 14kA. The breaker will be approved for use with a 35kA source. If you should happen to short wires while working near the breaker terminals it will probably not clear the fault. We found this out the hard way. This was a topic of interest in a past IEEE PCIC and IEEE IAS conferences where some folks tested this and photographed the results as part flew out of an open MCC bucket. Wire impedance can be significant.
If you use U/L labled molded case breakers check out what the interrupting rating really means. The U/L test method calls for about 10 feet of wire on the load side of the breaker during the test. The wire used will be the maximum size permitted for the breaker size. On the smaller breakers the 10 feet is sufficient to cut the fault from about 35kA down to 14kA. The breaker will be approved for use with a 35kA source. If you should happen to short wires while working near the breaker terminals it will probably not clear the fault. We found this out the hard way. This was a topic of interest in a past IEEE PCIC and IEEE IAS conferences where some folks tested this and photographed the results as part flew out of an open MCC bucket. Wire impedance can be significant.
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Uno,
Yes, discrimination is applied on almost all circuits, regardless of their purpose. It has to be, otherwise a fault on a small circuit would trip everything back to the utility transformer, if not back to the generating plant. This is achieved by discrimination on either a time or current basis on most LV systems. In addition to discrimation techniques, more specialised protection schemes are found on MV and HV systems and heavy-current LV systems, by restricting the zone over which the protection operates, or by making the protection sensitive to power flow in a certain direction, and so on. That is probably getting a bit too complex for this discussion, but protection is a fascinating field.
The use of cable impedance or transformer impedance to limit fault levels is standard practice. Fault level limitation by cable impedance is usually a fortunate result of using real-world cable sizes to supply real-world loads, where cable size is usually based on current-carrying capacity. Occasionally it is necessary to increase a cable to a larger size to withstand fault current, but while this is a prudent check for any design, the situation doesn't arise that often.
Transformers are deliberately designed with an impedance which restricts the prospective fault to a level which switchgear is cable of clearing. It is possible to design transformers with very low impedance, but switchgear becomes difficult to obtain or has to be designed for the application. In most situations the degradation in transformer regulation from no-load to full-load caused by increasing the transformer impedance slightly is of little or no consequence, while the benefits of limiting the fault level are apparent in reduction of conductor and switchgear size and cost.
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Start each new day with a smile.
Get it over with.
Yes, discrimination is applied on almost all circuits, regardless of their purpose. It has to be, otherwise a fault on a small circuit would trip everything back to the utility transformer, if not back to the generating plant. This is achieved by discrimination on either a time or current basis on most LV systems. In addition to discrimation techniques, more specialised protection schemes are found on MV and HV systems and heavy-current LV systems, by restricting the zone over which the protection operates, or by making the protection sensitive to power flow in a certain direction, and so on. That is probably getting a bit too complex for this discussion, but protection is a fascinating field.
The use of cable impedance or transformer impedance to limit fault levels is standard practice. Fault level limitation by cable impedance is usually a fortunate result of using real-world cable sizes to supply real-world loads, where cable size is usually based on current-carrying capacity. Occasionally it is necessary to increase a cable to a larger size to withstand fault current, but while this is a prudent check for any design, the situation doesn't arise that often.
Transformers are deliberately designed with an impedance which restricts the prospective fault to a level which switchgear is cable of clearing. It is possible to design transformers with very low impedance, but switchgear becomes difficult to obtain or has to be designed for the application. In most situations the degradation in transformer regulation from no-load to full-load caused by increasing the transformer impedance slightly is of little or no consequence, while the benefits of limiting the fault level are apparent in reduction of conductor and switchgear size and cost.
-----------------------------------
Start each new day with a smile.
Get it over with.
Uno,
Have you tried bussman's library for more info?
or directly for series rating...
Have you tried bussman's library for more info?
or directly for series rating...
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With respect to experience with the cascade approach, I have personally seen it applied successfully at in excess of 100 sites across a variety of industries.
In my 30 plus years of experience, most as a service engineer, I have not seen a single ciruit breaker failure that could be attributed to the fact it was not rated for the available fault duty and the upstream fully rated breaker failed to clear.
In my 30 plus years of experience, most as a service engineer, I have not seen a single ciruit breaker failure that could be attributed to the fact it was not rated for the available fault duty and the upstream fully rated breaker failed to clear.
uno, maybe I don't understand your problem but the 'short circuit rating' of a C/B is the amount of fault current it is rated to interrupt. This is a rating tested by the manufacturer and marked on the device. It appears you may be asking about fault current values being different at different locations, though this is true also.
Series-rated systems are based on an upstream device opening for a fault before the maximum fault current is reached. This technique can be used to 'limit' the fault current at a lower-rated downstream device, but it will only accomplish this by opening under the fault, which removes the selective coordination from the system(discrimination).
For instance, you may have a 400A fuse open the feeder to a 400A panel when the fault was on the load side of a 20A feeder C/B in the 400A panel. If the system was fully-rated, the 20A C/B would be designed to open and isolate the fault from the rest of the system. In a series-rated system, the whole 400A panel is de-energized.
Series-rated systems are based on an upstream device opening for a fault before the maximum fault current is reached. This technique can be used to 'limit' the fault current at a lower-rated downstream device, but it will only accomplish this by opening under the fault, which removes the selective coordination from the system(discrimination).
For instance, you may have a 400A fuse open the feeder to a 400A panel when the fault was on the load side of a 20A feeder C/B in the 400A panel. If the system was fully-rated, the 20A C/B would be designed to open and isolate the fault from the rest of the system. In a series-rated system, the whole 400A panel is de-energized.
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jbartos
Electrical
- Jan 15, 2000
- 6,440
Comment on uno (Electrical) Apr 27, 2004 marked ///\\I would like to know if anybody has used circuit breakers in a power distribution circuit where the upstream circuit breaker and the downstream circuit breaker ( maybe incomer to another distribution board) have different short circuit ratings.
///Often, the power distribution system physical geometry covering length between different panels, power distribution panels, switchboards, switchgear is such that one level downstream circuit breakers and buses (panelboards, MCC, switchboards, etc.) have lower short circuit level rating. If the buses are were close, then the short circuit rating will be necessary to be the same. The short circuit rating is a safety related since the protector may explode. It is somewhat different from the selective coordination since the protectors may have the good selective coordination and be improperly rated short circuit-wise thus causing a potential hazard. This area is often very poorly understood and in some instance poorly engineered and designed.
Nowadays, any more important or noticeable size electrical power distribution system is modeled by suitable software before it is constructed. The software normally includes short circuit and voltage drop analysis programs, e.g. SKM, Inc. DAPPER, A_FAULT, etc.\\There is no fuse used anywhere in the circuit.
///Good to know.\\
///Often, the power distribution system physical geometry covering length between different panels, power distribution panels, switchboards, switchgear is such that one level downstream circuit breakers and buses (panelboards, MCC, switchboards, etc.) have lower short circuit level rating. If the buses are were close, then the short circuit rating will be necessary to be the same. The short circuit rating is a safety related since the protector may explode. It is somewhat different from the selective coordination since the protectors may have the good selective coordination and be improperly rated short circuit-wise thus causing a potential hazard. This area is often very poorly understood and in some instance poorly engineered and designed.
Nowadays, any more important or noticeable size electrical power distribution system is modeled by suitable software before it is constructed. The software normally includes short circuit and voltage drop analysis programs, e.g. SKM, Inc. DAPPER, A_FAULT, etc.\\There is no fuse used anywhere in the circuit.
///Good to know.\\
jbartos
Electrical
- Jan 15, 2000
- 6,440
Comment on uno (Electrical) Apr 27, 2004 marked ///\\\
Or have you seen a power distribution circuit where the upstream circuit breaker is from a MCC/PMCC and feeds another distribution board which has outgoing breakers having lower short circuit ratings.
///Yes, this is quite normal if the distribution feeder is sufficiently long, i.e. has an adequate impedance to reduce the short circuit current (fault) down to the next standard interrupting rating level of a circuit breaker and its panelboard, switchboard, cabinet, etc.\\ There is again no fuse used anywhere in the circuit.
Or have you seen a power distribution circuit where the upstream circuit breaker is from a MCC/PMCC and feeds another distribution board which has outgoing breakers having lower short circuit ratings.
///Yes, this is quite normal if the distribution feeder is sufficiently long, i.e. has an adequate impedance to reduce the short circuit current (fault) down to the next standard interrupting rating level of a circuit breaker and its panelboard, switchboard, cabinet, etc.\\ There is again no fuse used anywhere in the circuit.
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- #14
Thanks, thanks to all..
PWR-It is a relief to know that this technology is being used somewhere in this world.With such an experience I would certainly like to consult you before pushing this application in my projects. Could you send me your email address at pandeys@mantraonline.com
Dandel- Your reply is very appropriate. I would like to add my views about my favorite topic IEC vs NEC. What I could gather that in the NEC world Series rating tests conducted by UL is such that panelboards/distribution boards are certified if both the incoming and the outgoing breaker trips on a short circuit in the outgoing feeder. However as you mention this can break power supply to the entire panel which has several outgoing feeders.In the IEC applications where UL listing is not required , supplier have started to gaurantee selective discrimination along with the series rating (also known as cascading).
jbartos- You certainly have a good view of the present philosophy that we engineers follow. Hoever I would like to bring about some change with increasing technological developments . Only one thing struck me. Why do you say that it is good to know that no fuse is being used?
Thanks again.
uno
PWR-It is a relief to know that this technology is being used somewhere in this world.With such an experience I would certainly like to consult you before pushing this application in my projects. Could you send me your email address at pandeys@mantraonline.com
Dandel- Your reply is very appropriate. I would like to add my views about my favorite topic IEC vs NEC. What I could gather that in the NEC world Series rating tests conducted by UL is such that panelboards/distribution boards are certified if both the incoming and the outgoing breaker trips on a short circuit in the outgoing feeder. However as you mention this can break power supply to the entire panel which has several outgoing feeders.In the IEC applications where UL listing is not required , supplier have started to gaurantee selective discrimination along with the series rating (also known as cascading).
jbartos- You certainly have a good view of the present philosophy that we engineers follow. Hoever I would like to bring about some change with increasing technological developments . Only one thing struck me. Why do you say that it is good to know that no fuse is being used?
Thanks again.
uno
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Some UL series rating info-
IEC cascade rating-
IEC cascade rating-
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davidbeach
Electrical
DanDel,
In your 4/29 post you state/imply that fully rated and selectively coordinated might be related. This is not at all true. What could be true is that a selectively coordinated system is fully rated, but the converse is not true. There are a number of examples where one can have both a fully rated combination of two breakers and a series rated combination where the manufacturer's trip curves are identical between the two. I could even put together the same thing using a fuse and a breaker.
In your 4/29 post you state/imply that fully rated and selectively coordinated might be related. This is not at all true. What could be true is that a selectively coordinated system is fully rated, but the converse is not true. There are a number of examples where one can have both a fully rated combination of two breakers and a series rated combination where the manufacturer's trip curves are identical between the two. I could even put together the same thing using a fuse and a breaker.
To expand on the previous post, a fully rated combination isn't necessarily any better coordinated than a series rated combination. Most MCCBs have an instantaneous trip region, which traditionally cannot be coordinated by definition. Some manufacturers apparently claim that certain combinations of their MCCBs can be coordinated.
davidbeach, what I said was that series-rated devices remove the selective coordination from the system. To have proper coordination, one of the first requirements is to have a fully-rated system.
Where did I say that all fully-rated systems are inherently coordinated, as you imply?
Where did I say that all fully-rated systems are inherently coordinated, as you imply?
jbartos
Electrical
- Jan 15, 2000
- 6,440
Comment on uno (Electrical) Apr 29, 2004 marked ///\\Hoever I would like to bring about some change with increasing technological developments . Only one thing struck me. Why do you say that it is good to know that no fuse is being used?
///A fuse can be current limiting. Such fuse can reduce the short circuit current, e.g. from 100,000A to about 15,000A (see Ferraz-Shawmut fuse AD2 or AD6). If such a fuse is prudently engineered and designed in the power distribution system, it will significantly reduce the short circuit current level downstream. Consequently, the downstream circuit breaker short circuit interrupting rating will be much lower, and so will be bracing of buses in panelboards, MCCs, switchboards, switchgear, etc.\\
///A fuse can be current limiting. Such fuse can reduce the short circuit current, e.g. from 100,000A to about 15,000A (see Ferraz-Shawmut fuse AD2 or AD6). If such a fuse is prudently engineered and designed in the power distribution system, it will significantly reduce the short circuit current level downstream. Consequently, the downstream circuit breaker short circuit interrupting rating will be much lower, and so will be bracing of buses in panelboards, MCCs, switchboards, switchgear, etc.\\
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