LSI Breaker - Question about the "S"

[designer82]

So table 430.52 is clear on Inverse time breaker being 2.5x FLA and also the Instantaneous setting being 8x FLA.

If I have an LSI breaker the "L" is the 2.5x FLA setting, the "I" is the 8x setting... What can I set the "S" to? (anything in between?)

Thank you

 

[mayanees]

So table 430.52 is clear on Inverse time breaker being 2.5x FLA and also the Instantaneous setting being 8x FLA.

If I have an LSI breaker the "L" is the 2.5x FLA setting, the "I" is the 8x setting... What can I set the "S" to? (anything in between?)

Thank you

Set for coordination with the upstream (and downstream) breakers. So it can be whatever the adjustments provide.

 

[xptpcrewx]

So table 430.52 is clear on Inverse time breaker being 2.5x FLA and also the Instantaneous setting being 8x FLA.

If I have an LSI breaker the "L" is the 2.5x FLA setting, the "I" is the 8x setting... What can I set the "S" to? (anything in between?)

Thank you

Both S and I are set with the goal of achieving coordination. You can refer to the IEEE buff book for guidelines on how to best set these. The instantaneous trip breaker column of Table 430.52 only applies to instantaneous trip breakers such as MCP’s, so you aren’t limited to a setting of 8X FLA with an inverse time LSI breaker.

 

[designer82]

So then as long as I meet NEC table 430.52 2.5x FLA requirement with the Long Time pickup, the short time and instantaneous I can set whatever I want. Did I understand that correctly?

 

[xptpcrewx]

So then as long as I meet NEC table 430.52 2.5x FLA requirement with the Long Time pickup, the short time and instantaneous I can set whatever I want. Did I understand that correctly?

Yes, however you need to set these elements such that it still complies with section 110.10.

 

[topgone]

So then as long as I meet NEC table 430.52 2.5x FLA requirement with the Long Time pickup, the short time and instantaneous I can set whatever I want. Did I understand that correctly?

As the term says "short-time" means whatever value you are expecting that falls between an "Instantaneous" short-circuit and the "long-time" (overload) situations of your load! Bluntly, it is a setting that you are allowed to set so as not to trip the controller on inrush or starting (as the case with motors)! My experience with this is when management decided to buy a replacement motor that is of higher efficiency! The starting current was higher (not over the instantaneous trip setting) but just over the "S" setting! We just have to bump the short-time setting a bit higher!

 

[paulengr]

Not sure about 2020 but previous NEC editions there are adjustable instantaneous trip breakers. You use one sized for the long term trip setting to 250% of FLA. Then set instantaneous to 11x. Increase if it nuisance trips but don’t exceed 17x.

Fully electronic tripping LSI breakers are rarely used in starters. Most common are MCPs and MMSs which are not technically circuit breakers because they don’t have LI which is the UL minimum requirement to call it a circuit breaker. If you are going to pay for electronic tripping a microprocessor based overload relay such as an SEL 710 or the Toshiba RX (actually an LSiS product) is vastly more useful for the money.

Realistically it will STILL trip on some high efficiency breakers. That’s where you need delayed instantaneous tripping (2-3 cycles).

Overall the goal is to trip on short circuits and provide a modicum of backup protection for overloads. Avoid inrush and locked rotor. The overload relay does the heavy lifting. Circuit breakers do not have a motor overload curve. Some MMSs do. Larger motors also require stall protection (rotor is more thermally limited than stator) which calls for a microprocessor based overload relay. The relay also offers several more “optional” protections such as ground fault, over/under voltage, over/undercurrent, often an adaptive starting trip function to minimize stalls, jam protection, current imbalance, and many others. Many MV contactors are used for interrupting short circuit current to save the motor fuses but also inhibit operation if current exceeds the contactor interrupting rating. Finally very large motors use a breaker instead of a contactor because there are none large enough.

It’s more intended for medium voltage but if you head over to the S&C Coirdinaide web site which is free if you know what time-current curves are you can chart a typical motor starting curve, overload relay, and breaker. This makes it very easy to visualize LSIG settings.