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]

jraef - Interesting. They sure did go backwards if they dropped the higher fault series ratings. I bet that's going to become a big issue for them as the arc flash requirements begin to have everyone looking at fault currents more closely.

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.

 

[Kiljoy]

Lionel,
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.

 

[LionelHutz]

Kiljoy - The testing procedure applies 100% to UL508A panels. The inspector will randomly show up for UL508A inspections so you could, in theory, ship anything you wanted between inspections. If the inspector did show up and look at the panel then he did not properly check the panels.

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.

 

[Kiljoy]

Allow me to clarify. I was referring to where the UL 508 code says that if a device is listed with a "High fault" rating (i.e. with fuses) then you can use a circuit breaker that allows for the same let-throug current even though the listing is with fuses and vice versa.

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.

 

[LionelHutz]

OK, yes you can change protection devices as long as you pick one with less let-through compared to the specified device. He already did that to change from RK5 to J fuses.

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.