Circuit breaker practical trip time under short circuit
Circuit breaker practical trip time under short circuit
(OP)
Hello,
I have a question regarding trip times of MCCBs under short circuit conditions in practical terms. This is to use in calculation for wire sizing as I need the amount of time the short circuit will take to clear. Even though the tripping curves for the breakers I have found indicate a trip will occur after 1 cycle or 20 ms for a 50Hz supply im not sure if I can take this as the time to clear the fault. Is it ok to use this as the fault clearing time for calculation purposes?
Thanks.
I have a question regarding trip times of MCCBs under short circuit conditions in practical terms. This is to use in calculation for wire sizing as I need the amount of time the short circuit will take to clear. Even though the tripping curves for the breakers I have found indicate a trip will occur after 1 cycle or 20 ms for a 50Hz supply im not sure if I can take this as the time to clear the fault. Is it ok to use this as the fault clearing time for calculation purposes?
Thanks.






RE: Circuit breaker practical trip time under short circuit
RE: Circuit breaker practical trip time under short circuit
Generally, it's not necessary to factor in fault clearing time for conductor sizing on circuits where you would use an MCCB. Those issues tend to be incorporated into code related conductor sizing requirements. But if you must know, you look at the total clearing time, which is often not necessarily clearly stated in an MCCB (again, because it's generally deemed unnecessary). So what you will see is the instantaneous trip SENSING time on the TCC curve, then you must find and add the mechanical unlatching action and opening times and then the arc extinguishing time. If you know a specific circuit breaker THAT YOU INTEND TO USE, someone at the mfr. level will be able to eventually find all of that information for you, but what you find for one will not translate over to another one, they are all going to be slightly different.
"Will work for (the memory of) salami"
RE: Circuit breaker practical trip time under short circuit
RE: Circuit breaker practical trip time under short circuit
I am using Australian Standards AS 3008.1.1 for cable selection up to 1kV, to specify cable sizes for a control panel build. I normally just focus on the control side of things but am now required to take some of the power and protection design work so I am a little out of my practical field of experience. Using the guidelines in the aforementioned standard there is a short circuit power calculation in order to size the conductors so they wont be damaged under the short circuit. This requires the prospected short circuit current, duration of short circuit and a constant K provided in a table. There are MCCBs being used to protect some of the branch circuits. In the case of the a fault on these branches after the breaker I believe I would need the breaker to also protect the conductors hence the need for this calculation.
Yes this is for the thermal protection of the cables. I will have a look at the data sheets again.
RE: Circuit breaker practical trip time under short circuit
I'm in Australia too. What size circuit are you trying to design for (in Amps)? You should really have an idea of fault levels (both max and min) before you size cables. If you can give me some more detail I could run a calc for you.
RE: Circuit breaker practical trip time under short circuit
Ill start by providing some context. The control panel is for a small piece of process plant, for mineral separation, to be designed and operated in house. The full calculated load on the incoming is around 35 A for 6 motors (2 VFDs, 4 DOL), a couple of instruments and some of the plant lighting. I have asked the electrical contractor that installed the distribution panels on site and they had no idea of the exact amount of fault current available only that the protection devices in the panel are rated to 6kA, which I was taking as the max. There are 6 single phase branches with rated loadings of about 0.5A to 2.5A, the two VFD motor branches have 19A and 4.7A input loads and the DOL motors are from 0.32A to 3.4A with manual motor starters as the protection device. Following the AS 3008.1.1 guidelines the low amp branches require a small x section with respect to current carrying capacity so I figure I need to size them based on short circuit considerations.
Thanks for any assistance you can provide.
RE: Circuit breaker practical trip time under short circuit
Firstly, if you are only feeding these small loads from an existing panel, it is likely based on what you have described that you are using MCBs (Miniture Circuit Breakers) not MCCBs (Mould Case Circuit Breakers) as stated previously. If a circuit breaker is rated at 6kA is is almost certainly an MCB. MCCBs are much larger and have higher ratings (usually at least 25kA).
If they are 60898 MCBs (as per BS EN 60898) a class 3, C Curve, 6kA (should be labelled with 6000 in a box with a 3 in the box under) device rated 20A to 32A should limit let-through energy to 52kA^2s at the high end.
A PVC 2.5mm2 cable can withstand about 77kA^2s (k=111, S=2.5). So should be OK.
Bear in mind the use of 6kA breakers does not mean the calculated maximum fault level is at or less than that. The MCBs may have been cascaded with a breaker upstream that is protecting the smaller breaker at a fault level above its native breaking level. When cascaded, the let through energy shown before, should still hold, but be sure to use the same make of breaker throughout.
If you are protecting the incomer to the board with a smallish breaker for the 35A load (maybe a 40A MCB?? although i'd go larger) it will be very difficult to achieve discrimination between that breaker and the ones feeding these small loads. A trip on one breaker may trip the upstream breaker too, taking out the whole board. This might be OK if nothing is critical and there are no safety circuits. There are rules of thumb in AS3000 for this, but they will unlikely produce a true discriminated solution.
Of course, none of this can really answer if the disconnect time requirement from AS3000 is satisfied. Again you could refer to AS3000 to add up impedances but that is a guesstimate at best but would cover your backside. To be strictly accurate you'd need to know all the cables back the the transformer.
If you are sizing the circuit breakers, don't forget to size the DOL motor breaker to 2.5x to 3x the running current to allow for starting current.
RE: Circuit breaker practical trip time under short circuit
I am aware some of the concepts of power protection such as coordination and discrimination but I suppose I lack the practical experience and understanding to be confident in the decisions I make. My studies are in mechatronic engineering, not pure electrical.
From a safety standpoint there is no real concern if the the trip occurs at the distribution panel breaker (there would if it goes any further that that as the dist board is feeding other equipment in the area) however there are some critical aspects from a process standpoint. Even though the plant is operated for a 3 to 5 days every few weeks if one particular circuit goes down at any time it will mean a couple or more tedious hours in maintenance trying to unblock pipelines and pumps and few $$ in material.
Thanks for the comment about the motor protection but already on top of that one.
I will check with the breaker datasheets we use and confirm their let though energies and size the cables that way. The current capacity calculations for the main circuit of 35 A I came out to 16mm^2 and the 19 A VFD circuit at 4mm^2.
RE: Circuit breaker practical trip time under short circuit
So in order to size the cables to account for protection against short circuit would it be enough to use the calculation you provided earlier so long as the branch protection device is enough to limit power let though during the fault?
As far as discrimination is concerned I suppose I will have to investigate what is up stream of the panel and dig further into the wiring rules to make a calculated decision.
RE: Circuit breaker practical trip time under short circuit
Yes, as far as I know, provided the thermal limit of the cable is greater than the let-through energy you should be OK. Bear in mind that to be complete you should check let-through across the range from min fault to max fault. Those let-through energies are for the maximum fault. If the minimum current was in the overload region you theoretically could get a higher let-through up to 5s.
For discrimination with breakers the size you are proposing, provided you aren't servicing any safety services, AS3000 2.5.7.2.3 would deem discrimination to be achieved if the ratio of the rating between successive breakers is at least 2x. So if you have a 50A breaker upstream, provided you use 25A or less in the control panel, you are OK per the standard. In reality that will probably only hold for fault currents up to about 400A. It is very difficult to achieve total discrimination between two MCBs.
RE: Circuit breaker practical trip time under short circuit
Conductor damage may occur in a control panel but it is invariably the result of a code violation or a failure of the protection devices.
I am not surprised that your contractor was unable to provide information on the damage curves of the conductors.
By "code" I mean, in North America, the National Electrical Code or in Canada the Canadian Electrical Code. These are the codes that contractors work to and AHJs inspect to. The code tables for the maximum ampacity of conductors and for the maximum size of breakers and or fuses provide protection against conductor damage.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Circuit breaker practical trip time under short circuit
In general, this is a dangerous assumption. At the very least, you should try to find the upstream transformer that is serving this system and get some idea of its size.
RE: Circuit breaker practical trip time under short circuit
dpc - I agree that in a perfect world the whole system should be included in the analysis. If there are existing IEC60898 6kA breakers installed on the site, I would consider the use of same (make and model) to be a defensible decision. At some point we need to assume that the person who came before did the right thing.
RE: Circuit breaker practical trip time under short circuit
healyx, there is some information I can find on Hager devices although very general and have not seen any figures in their documentation relating to a specific devices let-though energy. I suppose the next step would be to contact our suppliers or Hager themselves. As far as the evaluating the fault current available at the transformer I would believe this would be a question for the utility supplier?
Id like to assume that someone has done the right thing but seeing as I could not get any information about this from the contractor, whether or not they were the designer, invokes a little doubt and we know what happens when we ass-u-me. As you mentioned I would like to cover my behind should anything go wrong.
I have not had any specific training on this matter only what little power design I did at uni. I did attend a training workshop however it was more geared toward the high voltage substation level and did not cover much on end user equipment design. I think I may get some help until I can rely on some personal, practical experience. Or I could just be thorough in my own research and use of the relevant standards although this may take a lot of time.
RE: Circuit breaker practical trip time under short circuit
All you need to know is the kVA rating of the transformer, from that we can make a good assumption of the % impedance and thus the maximum fault level at the LV terminals. If you are in a industrial plant, I'd be surprised if you didn't have a dedicated transformer (or more). I'd also be a bit surprised if you didn't have as-built documentation showing the plant's single line diagram.
If you can get the transformer kVA and the make/model of the upstream circuit breakers, you'll have enough info to guarantee that the 6kA breakers are sufficiently rated. In fact the breaker feeding the board that feeds the control panel may be the only one you need to check. If cascading is achieved you are good to go. Will need to be the same make of breakers to check the cascading tables. Checking cascading will mean you don't have to worry about cable sizes (excluding disconnect time analysis).
I'd be surprised if you find it to be done incorrectly but kudos for being thorough. Most people would just install more 6kA breakers without giving it a second thought.
Don't worry about not knowing this stuff from uni, no unis seem to teach LV systems protection. I certainly didn't learn it there. If you are interested, I'm pretty sure NHP or Schneider would run courses on it.
RE: Circuit breaker practical trip time under short circuit
From the training workshop I did perform some example calculations and were given resources to estimate the transformer % impedance from its power and primary/secondary voltages to find the prospected fault current so if I had the information I would be comfortable. We operate in the metropolitan area but we are actually a registered mine site due to the amount of pilot scale mineral processing we do. We are in an industrial area so I don't think our facility we have a dedicated transformer. As a pilot testing facility we house a variety of smaller plants that are run on an as need basis for client requirements. We did have a larger plant installed that required more power than available and I believe that a new switchboard and larger cables were put in by the supply company, but I had no involvement with that.
If needed I will contact the supplier as I am sure they have all the information. As far as documentation that is kept on site, I am not sure.
This does raise another question, what about dedicated plant purchased from either in or outside Australia? As the manufacturers do not know for sure where there plant would end up how would they design protection systems? I suppose it would be up the purchaser to make sure they comply.
RE: Circuit breaker practical trip time under short circuit
RE: Circuit breaker practical trip time under short circuit
As it turns out there was a new substation transformer installed on our site to service our processing equipment and have managed to obtain the design drawings from the contractor. The transformer is a 630kVA/22kV to 415V but no listed % impedence rating. From the resources I have been given, an estimate can be made for %Z based on transformers of 0.25MVA, 0.5MVA, 1MVA up to 10MVA. The stated value of %Z for a 22kV primary voltage for a 0.5MVA is 4.5%. Does this seem accurate seeing as the installed transformer is 630kVA and not 500kVA? After that I would still need to assume the cable resistance at the control panel to calculate the actual fault current.
I did not get any information of the switchgear for the substation or main distribution board, only that the sub board feeding the plant I am designing will more than likely be a hager 6kA 50A breaker.
Thank you very much for all your input, it has been most helpful.
RE: Circuit breaker practical trip time under short circuit
RE: Circuit breaker practical trip time under short circuit
My calculation came out at 19.48 kA. I also tried to calculate the total resistance between the LV terminals and the distribution board in question to ensure a maximum fault of 6kA and came out to 0.027 ohms. Does this seem correct? Also the drawings provisions are that a 25kA SPD be installed to be fault limiting with trip setting at 500A. Im not entirely sure what this means? The fault current is limited to 500A or the SPD will trip at 500A?
RE: Circuit breaker practical trip time under short circuit
RE: Circuit breaker practical trip time under short circuit
RE: Circuit breaker practical trip time under short circuit
We usually multiply the fault level by a voltage tolerance factor of 1.06 in Australia.
To get cable resistance I assume you added up the cable resistances from AS3008? Some notes:
(1) You need to include reactance too. Larger cables usually have more reactance than resistance. Then add them all vectorially. Use X/R ratio of about 3 for the transformer.
(2) Don't use the resistance figures from AS3008 for maximum fault current calcs. They don't show for 20 degrees celcius which is where maximum fault levels are calculated.
I assume the 25kA SPD fault limiting refers to a short circuit protective device (circuit breaker). 500A would be the long-time trip setting, eg. the operational current that the device will pass without ever tripping. So if all the switchgear is Hager, I suspect they have used a 25kA rated 630A MCCB and set the trip to 0.8 x In to get 504A. As for what the fault current is limited to, that's a bit tricky to say. You need to calculate the fault current at the device location, then project it up to the line on the peak-current curve (from data sheets), then across to the normalising line (the one stepped with power factor), then back down to see the peak limited in rms. This method is rough and is unreliable for series connected fault current limiting breakers. I have never been advised to use this method but I've seen it done with fuses. A Hager h630 LSI breaker seems to limit rms current to 3.5kA peak or about 2.5kA rms. That seems surprisingly low to me. I'd go off the cascading table data (which are produced using actual tested combinations).
It is pretty unlikely that the "calculated" fault current will be less than 6kA at the DB. More important is to check the cascading table data between the upstream breaker and the 6kA breakers. By upstream, I mean upstream of the 50A MCB use are using. A 6kA 50A breaker will not protect other 6kA breakers obviously. You'll need to know the actual make and model of the breakers for this. I suspect you might have another MCCB between the 50A breaker and the 500A breaker, maybe a 160A.
RE: Circuit breaker practical trip time under short circuit
RE: Circuit breaker practical trip time under short circuit
It goes on to explain about co-ordination and that two breakers connected in series can clear a larger fault than the rated downstream breaker, due to the energy let through of the upstream breaker, as you have been saying, although it is referred to as back-protection. The only downfall is selectivity cannot be achieved at higher fault levels. From the back-up protection table it states that if a 250A MCCB is upstream of a 6ka MCB, the effective safe breaking capacity of the system downstream is 20kA, without causing damage to any of the devices, or 16kA for a 125A MCCB.
I will trace back and find the full breaker network from the transformer. There is the main switchboard (with the 500A breaker) feeding 3 sub-boards, one of which feeds the 50A breaker board. As far as the resistance calculation that is simply the minimum cable resistance required from the LV terminals to the 50A breaker to reduce the fault current to 6kA. Ill try to find the actual cables and sizes and at least provide an estimate based on the tables in AS3008 for cable resistance and reactance. Ill come back once I have this information and calculations and hopefully move on.
Thanks.
RE: Circuit breaker practical trip time under short circuit
Ok so I am still trying to decide if I can be reasonably confident now that the 6kA breakers to be installed will be protected through cascading with the fault level available. The MCCB in the distribution board feeding the smaller breaker is a Schneider NXS100B. I found the Schneider cascading table data for these breakers and it indicates that a C60N MCB (similar to hager 6kA breaker to 60898) with have a reinforced breaking capacity of 20kA rms. Also an estimation for cable resistances out of AS3008 I came up with a maximum fault level of 9.58kA at the main DB and according to the DB's paperwork the rated fault level is 10kA.
I know that the data must be for the same make of breakers but due to the reinforced capability of the downstream Schneider breaker being twice the prospected fault at that location, can I trust that this will be enough for the hager also. From the Hager data a similar MCCB will protect the Hager downstream device to 16kA.
Thanks.
RE: Circuit breaker practical trip time under short circuit
If calculated fault level is 10kA at the DB that houses the NSX100, isn't there another cable run to the DB that holds the 50A breaker and then another cable run to your new control panel. Is this the arrangement? If so, the board that holds the 50A breaker is probably down to at least 8kA, then you're running another 16mm^2 cable to your panel. It is pretty likely the fault level will be below 6kA at your new board, regardless of any upstream fault limiting, but to go down that route, you'd want to be sure that you have calculated the max. fault current properly. If you send me the cable types and lengths I can double check for you.
And no, you can't make any assumptions about cascading if the devices aren't tested in combination.
The NSX100B will cover any PVC cable over 7mm^2 for a fault between 4 and 21kA.
Regardless, given the fault limiting of the NSX100B and the cable impedance down stream, I think it will be fine. The 50A breaker already installed wasn't your doing anyway, but is probably fine too.
RE: Circuit breaker practical trip time under short circuit
So, you will either need to:
(a) Prove that the fault current is below 6kA at your control panel by calculation.
(b) Install a device that has been tested in combination with the devices you are planning on using. You could install a HRC (?100) fuse in your control panel to act as a fault limiter. Hager should publish the backed-up fault level for the HRC combination.
(c) You could to use schneider MCBs (C60N) in your control panel instead of Hager. We know that C60Ns are covered by the NSX100B breaker.
As for the Hager devices already installed under a Schneider breaker, they have either already installed a HRC fuse to firewall the fault level, or more likely not given it a second thought and just done it anyway....
RE: Circuit breaker practical trip time under short circuit
I do not have the exact lengths of the cables between boards, I made an estimate based on the physical distances between them and the current carrying capacity of the cables from the transformer and MSB are underground. I do know that the incoming cable to the NSX100B distribution board and there after are the orange PVC 4C+E power cables at about 25m of 70mm2 (MSB to DB) and 15m of 25mm2 (DB to 50A breaker board). I estimated between the transformer and MSB about 4m of 240mm2 cable and these are what I based my calculation on.
Ill have a look at the fusing option but I think I will just get the Schneider C60 installed as the MCB for this circuit. Can I used a fused isolating switch with the HRC fuses?
RE: Circuit breaker practical trip time under short circuit
RE: Circuit breaker practical trip time under short circuit
RE: Circuit breaker practical trip time under short circuit
RE: Circuit breaker practical trip time under short circuit
Data Used:
Tx to MSB. 240mm2, 4m, Cu XLPE, Single Core Trefoil.
MSB to DB1. 70mm2, 25m Cu XLPE, Multicore
DB1 to DB2. 25mm2, 15m, Cu XLPE Multicore
DB2 to Control Panel. 16mm2, Cu PVC Multicore.
With 630kVA, 4.5%, 415v Transformer, 500MVA upstream network, Cmax = 1.06. Max Fault Levels as follows:
Tx - 20.28 kA
MSB - 19.48 kA
DB1 - 14.41 kA
DB2 - 9.75 kA
CP - 7.98 kA
I don't think these cables sizes are correct, I think they would be larger in practice and thus fault levels would be higher (under ground in conduit 240mm2 only carries 426A...that transformer can supply 900A - seems weird)..
Anyway, I think you are well over the 6kA at the control panel and the board feeding the control panel. Provided you use C60 devices under that NSX100B you'll be fine up to 20kA and I am sure you are under that at DB2 and CP.