MCP & Fuses with a Soft Starter 3

[Astyanax]

Friends:
I'm just the CAD guy at a panel-builder, but am often required to size CBs and fuses to satisfy specs written by real engineers. We're to build a panel for a duplex submersible sewage pump station with 20hp (27FLA) Flygt pumps and Toshiba TE-39 (19-39A) soft starters. The engineer's spec calls for motor circuit protectors, and the customer wants a UL label, so I'm also using current-limiting fuses (class J) on each starter for a higher SCCR. Plus we'll have a circuit breaker to disconnect the panel.

The MCP rated for 30A has a trip setting for 27 FLA at 300A, even though with a soft starter the inrush can be programmed to 200%-300% of FLA. I want to use 60A fuses on each starter and a 60A main breaker (the ultrasonic controller will prevent simultaneous starting), but having all these different sizes looks incongruous and could be difficult to justify. Unfortunately, the obvious remedy - no MCPs - is not available. Is my sizing too small, or too large, or both at once? Thanx!

 

[LionelHutz]

40A fuses should be more than enough for pumps.

Make sure the 30A MCP is capable of handling 27A. Some breakers/protectors should only be run to 80% of their current rating.

The 300A rating sounds like the short circuit instantaneous trip level.

 

[jraef]

You will not be able to UL list that panel using MCP breakers by the way, unless the Tosh. RVSS units are specifically listed in combination with the specific MCPs you are using, which is a less than .0001% chance. MCPs are not UL listed, they are UL Recognized (UR) which means they can only be used in tested listed combination with other devices. In the case of MCPs specifically the testing includes destructive short circuit testing of the combination so is very very expensive, about $20K per line item last I checked, so nobody but the OEMs of the starter equipment can afford to do it and amortize it over a large volume.

but you can use regular thermal-mag circuit breakers, they are UL listed so you can use them any way you like. Most good ones now have adjustable mag trips anyway, so no need for an MCP, that was the difference they offered originally. Moot point anyway since the soft starter will preclude magnetizing inrush.



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[Astyanax]

Thanks guys!

Yes, the 300A is one of 8 cam settings for instantaneous trip, and the mfr. (Cutler-Hammer) recommends the 50A MCP for this size motor, presumably for DOL starting. I was actually more concerned about blowing the fuses in the event that the end user decides to program the starter for 300% or greater inrush.

As for UL, it seems that their principal concern is SCCR, hence the current-limiting fuses; we could do away with the MCPs altogether, but the customer's engineer spec'd the MCPs, not sure why.

 

[LionelHutz]

You're right Jeff, MCP or HMCP, aka magnetic trip or instanteous trip breakers are only UL listed when used in listed combinations. Typically, this requires use of the MCP, contactor and breaker from the same manufacturer. The MCP, fuses and a soft-starter stand little chance of being a listed combination.

Dou you mean time delay class J fuses? You won't blow those fuses sized at 40A on a Flygtt pump.

No offence meant, but the picking the right fuse and lack of achieving a UL listing on the panel when using that breaker are perfect examples of why a CAD guy shouldn't be putting this stuff together.

 

[Astyanax]

You're right Lionel, but what do you make of real engineers spec'ing MCPs with soft starters and VFDs? Or calling for said combinations to be UL-approved? Many end users are willing to pay extra for a UL inspector to place a seal of approval on such mongrels; for us, that usually means boosting the SCCR with current-limiting fuses.

 

[jraef]

Just because they have a title of "electrical engineer" doesn't mean they know everything about everything, this forum is a testament to the fact that we all need a little help now and then.

What I found happens most often with regards to soft starter specs is that the engineering firm this guy works for has a standard general spec for motor starters that includes using MCPs. Most engineers have little exposure to soft starters, but believe they are the same as any other starter in terms of circuit elements so they cut and paste the standard starter spec into the soft starter spec. What you end up with is a spec calling for something that does not actually exist in the real world. Common problem really. Usually if you just point out the issue they will allow the change without much fuss.

But did he spec MCPs AND fuses? That may indicate that he had used a soft starter spec from a specific manufacturer that requires fuses in order to satisfy their UL listing. That too is a common issue; its expensive to test soft starters with CBs alone and get a decent SCCR rating, so the short cut is to get the SCCR rating higher by requiring fuses up stream. Not all soft starters are like that however. If he want's a relatively generic spec so that anyone can bid it, then the fuses are typically over sized just to act as current limiters and the CB then does the normal every day SCPD, that way the fuses rarely clear.


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[Astyanax]

Thanks, jraef. The spec calls for UL508A certification for the panel, which in turn requires a 65kAIC rating for each branch circuit. For a panel with DOL starting, I could use a combination starter and CB that were each rated for 65k (and have done so in the past). But I could not satisfy the requirement with a soft starter and MCP, since, as you say, the two components would have to be rated as a unit after costly destructive testing.

So no, the spec didn't call for fuses; their only purpose is to satisfy the SCCR of 65k. The challenge is, as you say, to size the MCP or CB to trip without the fuses blowing.

Thanks again guys!

 

[LionelHutz]

Does the Toshiba have a combination listing approved for 65kA with class J fuses. And, as I already aluded to, which type of class J fuse? There are fast acting, time delay and high speed class J fuses.

You can not just randomly install current limiting fuses and get a higher fault rating. The device (ie the soft-starter) and the current limiting fuses have to be a listed combination just like the breaker has to be.

I didn't post that every engineer could do it either...

You should go to the general UL course on 508A and I believe it's a seperate short circuit course (but it might be together) if you want to learn about this stuff.

 

[jraef]

The Toshiba TS, TD and TX Series soft starters are actually made by Motortronics and they are listed behind Class J fuses at 85kA SCCR (I believe from memory). But I'm unfamiliar with a TE Series.


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[Astyanax]

Thanks again. I've inquired with Toshiba and Motortronics about documentation of the SCCR of their starters and VFDs, and with what protective devices those ratings were achieved. The MCPs on the branch circuits will have to be supplanted by CBs with a higher SCCR to match that of the fuses, and the main CB by either a CB with a higher SCCR or a fusible disconnect to limit the available fault current in the panel.

There turns out to be a good deal of literature available on the subject, most of it more informative than that supplied by our vendor, Ferraz Shawmut. Thank you for pointing me in the right direction.

 

[LionelHutz]

I hope it's just bad wording, because you don't install a CB or fuses to limit the available fault current in a panel.

The available fault current is basically dictated by the source and the wire inductance. A fuse or CB is not accepted as a way to lower it.

UL used to allow current limiting fuses to limit the fault current but it was often so badly misapplied that the practice is no longer acceptable.

 

[Astyanax]

Thanks, Lionel. So I ascertain the available fault current from the one line diagram, then use the fuse curve to determine the peak let-thru amps for my fuses, then determine whether the SCCR for the starter is exceeded. This starter's SCCR, according to Toshiba tech support, is 5k(!), nonetheless it appears that the peak let-thru at 100kA with the 40A Class J time-delay fuses is 2.5k instantaneous or 1k RMS.

And I should have said 'interrupting rating' when referring to CBs and MCPs, rather than SCCR. Do the MCPs default to 5k like a CB or are they entirely unrated? They could easily be supplanted by a fusible disconnect on each motor.

 

[LionelHutz]

You can not do that currve let-through current limiting crap. It appears you are still in way over your head.

The soft-starter and the fuses must have a combination listing which meets or exceeds your required short circuit capability

The soft-starter manufacturer says 5kA for the basic unit. You can not use that soft-starter on a circuit that exceeds 5kA unless there is a higher combination rating in the soft-starter UL file.

As an examle, the UL file might have a chart and in that chart would be the soft-starter model size and then it would list say Class J fuses and a maximum fuse size and a combination short circuit rating.

Once again, go take the UL courses if you want to do this.

 

[Kiljoy]

Here is a link to the SCCR ratings from UL for Square-D/Schnieder/Telemechanique/Toshiba soft starts. See if your product is listed in the sheet.

http://www.ul.com/global/documents/...ntrolpanels/shortcut/Square_D_Soft_Starts.xls

If you do not have, or can not find, a SCCR rating of the soft starter, then it defaults to 5KA. If the manufacturer publishes the rating with a specific upstream device, then use that value. If you can find it from the UL website that I linked above, even better.

One question: Above you state "The spec calls for UL508A certification for the panel, which in turn requires a 65kAIC rating for each branch circuit."

The UL 508 standard does not specifically require a 65KA rating. Is this the customer's requirement or are you assuming that it's a UL requirement?

 

[jraef]

Schneider and Toshiba share a common VFD manufacturing plant and some of their drive products are the same, but that is not the case with Soft Starters. Toshiba's soft starters are made by Motortronics, that's the only connection. Schneider's Medium Voltage soft starters are also made by Motortronics, but there is no connection between Motortronics and Schneider on Low Voltage soft starters.

The UL SCCR ratings are something that changed after I left Motortronics, but they unfortunately don't make it available on their website. I am inquiring about that for you with my old pals (those that remain after the recent bloodbaths), I'll post what I find. But I know for a fact that the information Toshiba's tech support people gave you is only with regards to stand-alone (non-combination) soft starters because that is the basic "courtesy" rating everyone gets on non-combination devices. Motortronics tested and listed them both in combination with Thermal Mag. circuit breakers and with Class J fuses. The UL "Withstand Ratings" ( the old term for what is now SCCR) changed with HP size when using breakers because of the different breaker let-through currents, but as I recall they were all listed at 100kA with the class J fuses. But I'll confirm if that's still the case because like I said, the rules changed after I left.

As to your use of MCPs, you as a panel builder will NOT be able to make any SCCR claims using MCPs because as I said earlier, they are not UL listed at all!


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[Astyanax]

Thanks again guys. I finally obtained the listing for the soft starter from Motortronics; it rates the 39 hp starter at 5kA when used with TRS (class RK5) fuses. Interestingly enough, Toshiba AS1 drives up to 50hp are also listed at 5kA when used with class J fuses. So the SCCR for the entire panel is 5kA, if I ditch the MCPs; otherwise it's zero.

And 65kA as a SCCR for the panel was based upon the IR of the main CB supplying the panel; the only way I could attain it would be to use VFDs with a higher rating on the branch circuits and a CB with a higher rating as the main disconnect. There are VFDs available with higher SCCR, achieved with class J fuses mounted internally.

 

[LionelHutz]

FYI, the 5kA is the minimum required for that HP range of devices. The device must pass the 5kA testing in order to be UL listed. It sounds like you are getting bad information sice the soft-starter is likely rated for 5kA with any suitable protection device and it could have a higher rating with certain fuse types.

 

[jraef]

No Lionel, I checked and that is what their UL report says.

Apparently nobody there truly understands SCCR ratings. They stopped the process after I left and nobody has picked it up to complete it, so it is left at the old "Withstand" values. I tried to tell them that they are missing the boat, but they were not open to listening. Coupled with the fact that they no longer pay me to care, I didn't press it further...


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[Astyanax]

Indeed, Lionel, the UL listing file rates the starters at the default values for starters of a given hp range, even with the specified fuses. I could substitute class J fuses for class RK5, since the peak let-thru current of class J, according to UL's Table SB4.2, is less than that of comparable class RK5; but it's still greater (8kA for 40A at 50-100kA available fault current) than the starter's SCCR, so the branch circuit's SCCR remains at 5kA.

Unless, that is, I can get away with using the fuse manufacturer's peak let-thru current and IR. I think that's why I've been passing UL - supplemental over-current protection on each branch circuit - even though my claimed panel SCCR's haven't been supported by documentation from Toshiba.