Non-Fused Disconnect Switch Inrush Rating

[tamiya0603]

Hello there,
I am new to the forum and are looking for help or opinion on the issue below from fellow electrical folks.

I have encountered a design issue with installing a 100A disconnect switch for a cooling fan application. The system is 3PH, 480V, Fan motor is 50HP 45FLA, it take about 3 sec for the fan to fully start and back to normal operating FLA. During the 3 sec inrush, we will see 240A. The 100A switch is installed near fan motor for isolation use only. during operation switch will stay closed and locked. Switch model number is a Westinghouse DS36U, based on datasheet it's rated for 75HP at 480V. i would think since switch is 75HP rated and it should withstand 75HP motor inrush. but most motor inrush last only about 1 sec, in our case the fan is taking longer about 3 sec to pick up full speed. there is no catalogue information mentioning inrush current rating or withstand durations.

I checked the UL98 standard of the switch also didn't mention anything about motor inrush. the UL98 standard did mention overload and endurance test but it's for make/break type of test where you flip the switch open/close. i sent the question to Eaton asking about inrush rating and duration rating but doesn't have any luck.

1. In my case i want to know if this switch can handle frequent motor start/stop and would the frequent inrush damage the switch? the client is concerned about switch will fail open and cause operation delays.

2. Response from Eaton is they cannot guarantee the switch would work indefinitely and recommended upsize or use a fused disconnect. i am looking for ideas or opinions on how to prove switch can withstand longer motor inrush.

3. The Eaton Technical Data 29-420 did mention 1800A interrupting duty and my understanding is interrupting duty is only rated for 3 cycles. which is about 0.05 sec. is there any way to prove switch will withstand 250A for 3 sec?

4. looking at the photo, the switch is very robust with thick metal contact pins, I am very confident to say the switch can withstand 240A, but there is not document to support that. based on UL98 overload and endurance test section:
In UL98 table 17, for a 75HP switch, the overload test at 480V is required to be 543A for 50 making/breaking cycles at 6 cycles per minute.
In UL98 table 19, 100A switch is required to have 10000 "with/without current" test cycles, if flipping switch open/close counts as 2 cycles, my interpretation would be the switch is good for operating 5k times.

thanks for all your help and comments.

 

[David Castor]

The disconnect switch must have a 50 hp horsepower rating for this application. If it is a NEMA rated switch that has an adequate horsepower rating, it will be able to withstand the inrush current with no concerns. Focus on the horsepower rating and not the current ratings. I also strongly recommend a fused disconnect switch for this application, as opposed to non-fused.

 

[tamiya0603]

The disconnect switch must have a 50 hp horsepower rating for this application. If it is a NEMA rated switch that has an adequate horsepower rating, it will be able to withstand the inrush current with no concerns. Focus on the horsepower rating and not the current ratings. I also strongly recommend a fused disconnect switch for this application, as opposed to non-fused.

Thanks for your response David, The DS36U is a 75HP rated switch it’s UL98 rated, I checked NEMA KS-1 standard with is identical to the UL 98 standard. For non fused disconnect switch, I consider the overload current test in UL98 is a way of testing motor inrush scenarios on the switch, for a 75HP rated 100A switch, 534A is injected for 50 cycles and switch passed the test. To me this switch should withstand 240A inrush. I also checked a lot of non-fused disconnect catalogue but couldn’t find anywhere mentioning how long switch can withstand inrush current. The history behind this application was previously site used a 50HP 80A ABB OT100F3 disconnect switch, upon start ups, the disconnect switch burned out and switch failed open. It leads to the conclusion that switch is undersized. So we went with 75HP instead. I would think non-fused and fused switch are built the same way with addition of fuse holder. Adding time Delayed fuse to avoid fuse blown during inrush. But the switch mechanism still sees same amount of inrush current regardless of fused or non-fused types.

 

[gar]

211112-1921 EST

tamiya0603:

You are talking about starting current, and I don't call that inrush. I would call inrush something that lasts more or leas within the first 1/2 cycle, and at 60 Hz 1/2 cycle is 8.3 milliseconds. Inrush occurs when you switch a tungsten bulb, or power a transformer, or charge a capacitor.

When you first close a mechanical switch there is for a short moment arcing, and a higher power dissipation in the contact area with high power loss right at the switch contacts. There is also a small amount of contact material transfer from one contact to the other. In a DC switched circuit this transfer takes place in a unidirectional direction, and you will see a mound of metal on one contact and a cavity on the other contact as the number switch changes increase. Thus, short contact life. A problem in automotive electromechanical voltage regulators, about 1000 hours of life. In an AC switched circuit the metal transfer occurs in both direction, and life is much greater,

In your case if the switch remains closed virtually all the time, then there is virtually no metal transfer, and power dissipated in the contact area is very small. Unless you go to very large switched currents there won't be any contact welding. If your switch is only switched when no current flows, then there is no metal transfer.

The above is the physics of the problem. Code may require you to do something different.

.

 

[David Castor]

Thanks for your response David, The DS36U is a 75HP rated switch it’s UL98 rated, I checked NEMA KS-1 standard with is identical to the UL 98 standard. For non fused disconnect switch, I consider the overload current test in UL98 is a way of testing motor inrush scenarios on the switch, for a 75HP rated 100A switch, 534A is injected for 50 cycles and switch passed the test. To me this switch should withstand 240A inrush. I also checked a lot of non-fused disconnect catalogue but couldn’t find anywhere mentioning how long switch can withstand inrush current. The history behind this application was previously site used a 50HP 80A ABB OT100F3 disconnect switch, upon start ups, the disconnect switch burned out and switch failed open. It leads to the conclusion that switch is undersized. So we went with 75HP instead. I would think non-fused and fused switch are built the same way with addition of fuse holder. Adding time Delayed fuse to avoid fuse blown during inrush. But the switch mechanism still sees same amount of inrush current regardless of fused or non-fused types.

I've never seen or heard of a properly sized NEMA HP rated disconnect switch failing due to locked rotor current. It's possible that the switch wasn't properly closed causing overheating and arcing during starting, or was in some other way defective. Not sure you can assume the switch was not capable of withstanding the starting current. Also, how old was the switch and what was its condition of maintenance?

Regarding fused vs unfused disconnnect swithces. The concern is the short-circuit rating in case of a fault downstream. A NEMA-rated unfused switch is rated for a maximum through-fault current of 10,000 A. AFAIK, there are no exceptions to this. The addition of a current-limiting fuse will increase the short circuit rating of the switch/fuse assembly to 100 kA or 200 kA.

Good luck.

 

[Jraef]

You are talking about starting current, and I don't call that inrush. I would call inrush something that lasts more or leas within the first 1/2 cycle, and at 60 Hz 1/2 cycle is 8.3 milliseconds. Inrush occurs when you switch a tungsten bulb, or power a transformer, or charge a capacitor.

I agree with this, but when I have said this here in the past, there are others who will argue that "inrush" has come to mean starting current. I disagree, but it's out there.

tamiya0603​

You are not going to find any definitive extended time frames on starting current capability of disconnect switches, but for contactors the design criteria is based on "normal" starting duty where acceleration takes place within the time frame of an Overload Relay trip time, i.e. a Class 20 OL would have to trip in 20 seconds at 600% current, so a NEMA contactor would have to handle that. You could INFER, by extension, that if you have a Combination Motor Starter with a Fused Disconnect Switch with an equivalent motor HP rating as the NEMA motor starter, that it would be capable of at least the same amount, otherwise the use of that switch would be a weaker point of failure than the other elements.

As to the Duty Cycle, NEMA motor starters are designed around the typical Duty Cycles of the motors at the top of the range of the NEMA size. So if the Starts-per-Hour rating of the motor is being exceeded, you would over size the motor to handle it, so then you would over size all of the control equipment to match the motor. So if their duty cycle fits within the Starts-per-Hour rating of their 50HP motor, then anything connected to it should survive too. If you are using IEC components, you are on their own.

 

[tamiya0603]

I've never seen or heard of a properly sized NEMA HP rated disconnect switch failing due to locked rotor current. It's possible that the switch wasn't properly closed causing overheating and arcing during starting, or was in some other way defective. Not sure you can assume the switch was not capable of withstanding the starting current. Also, how old was the switch and what was its condition of maintenance?

Regarding fused vs unfused disconnnect swithces. The concern is the short-circuit rating in case of a fault downstream. A NEMA-rated unfused switch is rated for a maximum through-fault current of 10,000 A. AFAIK, there are no exceptions to this. The addition of a current-limiting fuse will increase the short circuit rating of the switch/fuse assembly to 100 kA or 200 kA.

Good luck.

it's a brand new installation, there are more than one fan motors failing which leads to the suspicion of 50HP rated disconnect switch is under sized and won't be able to withstand the starting current of 240A for 3 second. i am now asking the vendor to confirm based on section 7.3.1.2 (c), a 75HP switch should have a current rating based on 60% overload current which is 326A. the 100A rating is only for continuously running load. As mentioned from you and other fellow responses, it would be a better design to go with the fused disconnect switch instead of a non fused switch.

 

[tamiya0603]

I agree with this, but when I have said this here in the past, there are others who will argue that "inrush" has come to mean starting current. I disagree, but it's out there.

tamiya0603​

You are not going to find any definitive extended time frames on starting current capability of disconnect switches, but for contactors the design criteria is based on "normal" starting duty where acceleration takes place within the time frame of an Overload Relay trip time, i.e. a Class 20 OL would have to trip in 20 seconds at 600% current, so a NEMA contactor would have to handle that. You could INFER, by extension, that if you have a Combination Motor Starter with a Fused Disconnect Switch with an equivalent motor HP rating as the NEMA motor starter, that it would be capable of at least the same amount, otherwise the use of that switch would be a weaker point of failure than the other elements.

As to the Duty Cycle, NEMA motor starters are designed around the typical Duty Cycles of the motors at the top of the range of the NEMA size. So if the Starts-per-Hour rating of the motor is being exceeded, you would over size the motor to handle it, so then you would over size all of the control equipment to match the motor. So if their duty cycle fits within the Starts-per-Hour rating of their 50HP motor, then anything connected to it should survive too. If you are using IEC components, you are on their own.

Hello Jreaf,
everything in the MCC bucket is properly sized per 50HP motor and the disconnect switch at fan motor end is 75HP rated. just got a response from Eaton saying the UL98 overload test does not apply to non-fused disconnect switches which to me is a surprise, i would think all switches go through overload test. Eaton Tech is also saying DS36U switch will not fail open unless it's opened manually. It contradicts with the ABB switch which failed open during motor starting (see attached photo of a typical ABB switch). i guess the ABB switch is built differently where the plastic casing breaks, the spring holding metal pins will release and loosing contact.