Circuit breaker practical trip time under short circuit

[kallen88]

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.

 

[kallen88]

Cheers,

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?

 

[stevenal]

The impedance is measured after construction, so it doesn't show up on the design drawings. The impedance should be stamped on the nameplate and can also be found in the test reports.

 

[dpc]

The available fault current is something you don't want to assume due to the hazard involved. It is unfortunately very common to find circuit breakers not adequately rated for the available fault current - at least in my corner of the world. It happens all the time. So even if you do need to make assumptions regarding fault current, basing it on the SC ratings of the existing breakers is not the way to go.

 

[healyx]

Kallen88,

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.

 

[healyx]

I'm pretty sure that peak let-through current from the previous post is wrong. I think the Hager curve might be wrong, it doesn't seem to correspond with the let-through energy data and trip curve. I think the limited rms current for a 20kA fault would be closer to 15kA rms, not 2.5kA rms. So definitely need cascading data to prove the combination.

 

[kallen88]

Ok I think I am finally starting to understand how everything fits together. I read through a technical document from Hager explaining all the concepts you are talking about with some nice examples. It also cleared up some other concepts I knew about but did not fully understand such as where the DC component comes from in an asymmetrical fault.

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.

 

[kallen88]

Hello again,

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.

 

[healyx]

Looking at the current-limiting curve for a NSX100B, a 20kA rms fault is limited to 8kA rms. If you are saying the maximum fault level at the location this NSX100B is installed in 10kA, the NSX100B will limit this to 6kA (only just). As I said before, I don't really know if this is a valid method.

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.

 

[healyx]

Just done a search, the method where you calculate the limited rms current (Up-over-down) method has been disproved where you are trying to protect a current limiting breaker down stream. I couldn't find any IEC references for this but it is accepted to NOT use this method in North America (NEC) and makes sense to so the same in IEC jurisdictions.
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....

 

[kallen88]

Ok Thanks for all your help. Hope I havnt taken too much of your time, this has been a rather long thread.

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?

 

[healyx]

If you are going to use Schneider C60Ns throughout (including for the 50A breaker), you don't need the HRC fuse(s). Provided you checked the cascade table under the NSX100B, you are good to go. But yes, you can get fused HRC isolators. I'll do a calc for you later and post the results.

 

[healyx]

What is the distance from 50A breaker board to your control panel? Is everything 3 phase?

 

[kallen88]

The distance between the 50A breaker and control panel is about 6m of 16mm2 cable and yes it is all 3 phase. Thanks.

 

[healyx]

I did a calc.

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.