MCP & Fuses with a Soft Starter
MCP & Fuses with a Soft Starter
(OP)
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!
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!





RE: MCP & Fuses with a Soft Starter
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.
RE: MCP & Fuses with a Soft Starter
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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RE: MCP & Fuses with a Soft Starter
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.
RE: MCP & Fuses with a Soft Starter
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.
RE: MCP & Fuses with a Soft Starter
RE: MCP & Fuses with a Soft Starter
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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RE: MCP & Fuses with a Soft Starter
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!
RE: MCP & Fuses with a Soft Starter
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.
RE: MCP & Fuses with a Soft Starter
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RE: MCP & Fuses with a Soft Starter
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.
RE: MCP & Fuses with a Soft Starter
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.
RE: MCP & Fuses with a Soft Starter
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.
RE: MCP & Fuses with a Soft Starter
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.
RE: MCP & Fuses with a Soft Starter
h
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?
RE: MCP & Fuses with a Soft Starter
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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RE: MCP & Fuses with a Soft Starter
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.
RE: MCP & Fuses with a Soft Starter
RE: MCP & Fuses with a Soft Starter
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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RE: MCP & Fuses with a Soft Starter
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.
RE: MCP & Fuses with a Soft Starter
Astyanax
You have demonstrated numerous times that you do not understand the fault testing or rating numbers. The short version of how the fault testing is performed goes like this.
The cables being connected to the device under test are shorted together.
The impedance of the fault generator is adjusted to get the desired test current (say 5kA).
The device is then connected and tested.
The big thing to note is that actual current that flows during the test is not 5kA. 5kA would flow if the leads at the incoming to the device are shorted. Once you add the breaker or fuses and the device and the output wires (which are shorted) then the actual test current will be less than 5kA.
So, you can not use the 5kA rating and compare that rating to a breaker or fuse let-through curve. You would need to have the actual measured current that flowed during the fault test.
This is why I posted before that using the let-through current is not valid. Quit thinking about finding a fuse or breaker with a let-through less than 5kA.
RE: MCP & Fuses with a Soft Starter
I understand what you are saying about the testing procedure, but Astyanax mentioned that the panel needed to be UL508A. That's a bit of a different beast. When you are a certified UL508A builder, you build panels and do not submit them to UL for destructive testing. You certify that you build the panels to the UL508 standard and submit to quarterly inspections and are required to use all UL listed components. With my personal experience, the UL inspectors do not always have the same knowledge as electrical designers and engineers. The reason Astyanax probably squeeked by is that the inspector didn't know any better. My inspector keeps telling me to de-rate NEMA 12 enclosures to NEMA 11 because I use NEMA 4 components.
Astyanax
The being said The 508a standard does indeed let you rate the SCCR of your components higher if you use current limiting fuses with the same peak let-through as the recommended device (High Fault Rating). But, if Toshiba says 5kA WITH fuses, then either rate your panel 5ka or get another soft starter.
I do find in a little odd that you are using supplemental protectors in place of branch circuit rated devices. This could be a big problem.
RE: MCP & Fuses with a Soft Starter
Your last sentence is not very clear.
You can not create a higher SCCR panel simply by picking a protective device with a let-through current lower than the downstream component rating. In this case, picking a fuse with a let through current less than the soft-starter 5kA rating is not a valid way to increase the SCCR above 5kA.
You can install a upstream protection device which limits the current to the complete combination of devices. In this example, the J fuses and soft-starter combine to give a 5kA package. You could install a second set of fuses (or a breaker) upstream of the J fuses IF those fuses (or breaker) limit the let-through to 5kA. In this manner, the fault current reaching the 5kA rated combination of fuse and soft-starter.
Astyanax - Doing the above is very different than what you have been indicating. You indicated that you were picking a fuse to limit the current to the 5kA rating of the soft-starter. In other words, a single fuse per phase in front of the 5kA rated soft-starter would increase the fault rating to a higher value. This is not valid.
A set of fuses limiting the current to 5kA in front of the 5kA rated combination of fuses and soft-starter is a valid method to increase the fault rating.
RE: MCP & Fuses with a Soft Starter
If what Astyanax said is true, and the soft start is rated for 5KA with fuses, then (as you said) his only choice is to reduce the avilible fault current via upstream devices or get a SS with a higher rating.
RE: MCP & Fuses with a Soft Starter
The original posting with a MCP breaker and fuses in front of each soft-starter to increase the soft-starter fault rating was completely wrong.
A circuit breaker with <5kA let-through and fuses in front of each soft-starter could be applied. This is likely not possible with a suitably sized breaker though.
Kiljoy - one other comment, the OP was refering to a MCP (HMCP) which looks more or less the same as a thermal magnetic breaker except it only has a magnetic trip.