Molded Case Circuit Breaker Trip Units, Types and Applications
A fundamental element of all low voltage circuit breakers is the trip unit or ‘brain’ of the circuit breaker. Several different trip unit technologies are available, but which one is the best choice for my application? This presentation, given by Andrew Legro P.E. at ABB, provides an overview of each trip unit technology from thermal magnetic to multi-function digital. Cost versus function will be reviewed and as well as a walk-through of some example applications.
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Answers to Q&A Following Live Webinar
|Question||Answer (Andrew Legro at ABB)|
|Can these breakers TCC settings be adjusted while on-line or will an outage need to occur?||Setting adjustments can be made while the breaker is energized, closed, and within rated load. Care must be taken because if the new setting is lower then the load, the breaker will trip. The Bluetooth communication option available in Ekip Touch trip units is useful for this situation as the operator can stay clear of the arc-flash distance while changing the breaker settings.|
|Are the XT breakers compatible with older GE Spectra panels, i.e. will an XT breaker fit in a Spectra panel.||Yes, ABB manufactures a retrofit kit to install new Tmax XT breakers into existing Spectra bolt-on panels.
|Please discuss listing of breakers that have been retrofitted with a trip unit. Can the trip unit be replaced on a UL listed breaker without type testing?||ABB trip unit conversion kits are design verification tested to ANSI C37.59 but do not carry a UL certification mark. The UL product category PAYK certification covers the retrofit trip only and not the breaker onto which it is installed. Primary injection testing is typically performed on all trip unit retrofit breakers to ensure proper operation.
|What is the difference between a 3-pole LSIG breaker and 4-pole LSIG breaker?||A 3-pole LSIG breaker can be applied to either a 3 wire (no neutral) or a 4 wire (solid neutral) system. For a 3-wire system, ground fault current is measured by summing phase current vectors A,B,C. For a 4-wire system, an external neutral CT is installed and the ground fault current is measured by summing current vectors A,B,C,N.
A 4-pole breaker always measures ground fault current by the sum of A,B,C,N. The breaker also includes neutral overcurrent protection. On ABB Ekip Touch trip units, the neutral overcurrent protection can be set to 50%, 100%, 150%, or 200% of the phase overcurrent settings.
|In the time current curves of MCCB's can we see neutral protection curves?||EasyPower does not include separate settings for neutral overcurrent protection in molded case or power circuit breakers. Typically, the neutral protection is set to match the phase overcurrent settings so there would be no need for a separate neutral curve. In cases where neutral protection settings differ from phase, I suggest copying the breaker and pasting it directly in series then use this breaker for the neutral settings and curve plot.|
|Can you explain the difference between Icu, Ics, Icw which are written on the label of MCCB?||These terms are part of the IEC standard so they don’t apply to UL-489 listed breakers. The ABB Tmax XT product line is however a global breaker and meets both IEC and UL standards. Sometimes the IEC ratings are printed on the ABB breaker in addition to UL for breakers sold in the USA.
My understanding of these definitions (per IEC 60947-2):
Icu: Ultimate breaking capacity in kA RMS. The breaker will successfully interrupt this fault current twice during a (trip – wait – close & trip) sequence. After this sequence, the breaker is not required to carry it’s rated current and needs to be replaced. The UL-489 nearest equivalent term is Interrupting Rating.
Ics: The rated short circuit capacity in kA RMS where the breaker can successfully interrupt three times in a sequence of (trip – wait – close & trip – close & trip). After this sequence, the breaker is required to carry it’s rated current. Typically, this rating is given as a percentage of the Icu rating. This value ranges from 25% to 100% of Icu. No UL-489 equivalent term exists.
Icw: Withstand capacity in kA RMS. This is the maximum current that the breaker carry for a specified period of time without being damaged. The withstand time is specified by the manufacturer but is typically either 0.05, 0.1, 0.25, 0.5 or 1 second. No UL-489 equivalent term exists as UL-489 breakers do not have a withstand rating. UL-1066 listed breakers (LV power circuit breakers) do have a 30 cycle withstand rating.
|Do ABB MCCB's use Rogowski coils as current sensors instead of iron core current transformers?||Both Rogowski coils and iron core current transformers are used in the ABB Tmax XT when equipped with the Touch / Hi-Touch trip unit.|
|Which ABB molded case circuit breakers include thermal memory?||All ABB Tmax XT molded case circuit breakers with solid- state trip units include thermal memory. The Tmax XT breaker line replaces the legacy GE Spectra RMS that did not include thermal memory.|