Control PCB of a circuit breaker trip unit: the electronics that measure phase current and trip on long-time, short-time, instantaneous and earth-fault settings

01

Long-time protection: an I²t curve with thermal memory and a 1.05–1.20 Ir trip band

Schneider Electric describes long-time protection on its ComPacT NSX MicroLogic 5, 6 and 7 trip units as I²t inverse definite minimum time protection that incorporates a thermal image, set by the Ir pickup and tr time delay. Ir is preset with a dial and fine-tuned on the keypad, or set by password-protected software or a network command; its default is the maximum dial position, In, and under IEC/EN 60947-2 the long-time tripping range is 1.05–1.20 Ir. On a 250 A trip unit the dial presets start at 100 A. [1]

Schneider's MicroLogic 2 and 4 units set Ir on a multi-position dial whose maximum equals the trip unit rating In; the MicroLogic 4, which adds earth-leakage protection, has a switch that disconnects the trip unit supply from the phases for panel dielectric testing. [2]

Give the supplier the breaker frame and current ratings, the protection functions needed (L, S, I, G, earth leakage, zone selectivity), how settings are entered (dials, keypad, software), the trip actuator interface, communication needs, the applicable standard (IEC/EN 60947-2) and the tests to be run on each board and on the assembled trip unit.

02

LSIG thresholds, delays and tolerances on one trip unit's settings table

ABB's SACE Emax 2 catalogue gives the settings of its Ekip Touch, Ekip Hi-Touch, Ekip G Touch and Ekip G Hi-Touch trip units, showing the range a control board has to resolve and time for each function. [4]

The same table lists optional IEC inverse curves for L (standard, very and extremely inverse), zone selectivity with t2sel of 0.04–0.2 s, current unbalance, and closing-on-short-circuit protection. [4] Editorial calculation: at I1 = 0.4 × In on a 1,000 A sensor, the overload pickup is 400 A and the trip band 420–480 A; the I3 setting at 15 × In is 15,000 A, so the board's current measurement has to span from below 400 A to above 15 kA. [4]

03

Secondary injection checks the electronics; primary injection checks sensors and wiring too

Schneider's field testing guide for MicroLogic circuit breakers measures long-time delay by applying 300% of the ampere rating and timing to trip, and notes that the UTS3 secondary injection test kit can run the long-time test but does not test the current transformers and connections. Short-time pickup levels are multiples of sensor size times the rating plug multiplier, and the short-time function can be tested with a high-current, low-voltage AC supply; the guide says not to run the contact resistance test before instantaneous primary injection testing, which clears resistive films from the contacts. [3]

Editorial note: for the control board itself, ask the supplier which tests run on the bare assembled board (in-circuit or functional fixtures), which on the finished trip unit by secondary injection, and which need primary current through the breaker's sensors.

04

Send the breaker ratings, protection functions and tests together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Breaker: [frame; In; sensor ratings]
Functions: [L; S; I; G; earth leakage; zone selectivity]
Settings: [dials; keypad; software; steps]
Tolerances: [pickup; timing]
Interfaces: [trip actuator; communication; supply]
Tests: [board functional; secondary injection; primary injection]

See manufacturer records Send an enquiry through CNFX