What is Correct Method of MCB Connections


Introduction:

  • MCB is a mechanical switching device which can carry and break currents under normal circuit conditions and also under specified abnormal conditions, such as overload and short circuit.
  • The MCB can provide protection until and unless we have install input power (LINE) connection and Output (LOAD) connections in proper Terminals of MCB.
  • Electrical engineers seem to be confused to indentify where is the Line and Load terminal of an MCB (on the top or on the bottom).

Terminal Marking of MCB:

  • There are two type of MCB available in market.
  • MCB having terminal marking (LINE / LOAD Marking) (Polarized MCB)
  • MCB having No terminal marking (No any Marking) (Non Polarized MCB)
  • Some manufacture clearly indicates where to apply Input Power and where to connect Load on MCB while some manufacture does not indicate such Terminal Marking.
  • The constructions of both MCB are almost same even though we need to understand difference between them.

(1) LINE / LOAD Terminal Marking on MCB (Polarized MCB)

  • For AC Circuit:
  • If manufactures indicate Input (LINE) making on MCB then we have to give Supply at “LINE” Terminal and Load at “LOAD” Terminal for perfect operation of MCB.
  • If we do wrong connection than MCB may or may not give proper protection in fault Condition.
  • As Per UL 489 Paragraph 9.1.1.13: It is clearly indicate that “Circuit breakers shall be marked “Line” and “Load” unless the construction and test results are acceptable with the line and load connections reversed. This marking requirement specifies that UL MCB shall be marked with the word “Line” on one end of the circuit breaker and the word “Load” on the other end”, as shown in Figure

111

  • If MCB is not live (ON) from long time (in Cold state) than there is possibility of MCB to not operate in fault conditions.
  • In MCB ,The fixed contact is encompassed by the arc chute, and the arc products are de ionized, cooled and ejected uneventfully when the incoming power is on “Line” Terminal (when the fixed contact is ‘live’ or ‘hot’).There is less chance to re strike arc again.
  • If the power is applied to moving contact ,”Load” Terminal, the flexible connector, the trip system, everything is live/hot after the arc is quenched. Chances of restrike/flashover are much higher.
  • For DC Circuit:
  • The polarized DC MCB have a marking of ‘+’ and ‘–‘ symbol
  • If Polarized DC MCB are wired incorrectly, they are a possibility of hazard and When we turned off under load, the MCB might not be able to extinguish the arc and the circuit breaker will burn out.
  • Polarized DC MCB use a small magnet to direct the arc away from the contacts and up into the arc shoot and arc disrupter cage. If the direction of current flow through the unit is reversed, then the magnet directs the arc away from the arc shoot and into the mechanism of the unit thus destroying it.

2

(2) No Terminal Marking on MCB (Non Polarized MCB)

  • For AC Circuit:
  • If manufacture has not indicated any Terminal Marking than we are free to connect line or load at any side as we wish.
  • If construction / Operating principle of both MCB are same then what are the different between them.
  • Without Terminal Marking MCB has following additional features.
  • (1) By Design improvement (Manufacture has provided some more provision for quenching of arc (So it cannot reproduce it again).
  • (2) By doing some more extra test as per IEC 60947-2 and UL 489

3

  • The performance of single-break circuit breakers is slightly different when the “LINE” and “LOAD” feed either from the bottom or Top hence IEC 60947-2 specifies that one additional SC test be carried out with connections required when the terminals are not specifically marked ‘Line’ and/or ‘Load’

Table 10- Number of samples for test (IS / IEC 60947-2)

Test Sequences

Terminal Marking (Line / Load) No of Sample for Testing

 

Sample For *

 

YES

NO

 In

1 1
Ics (Rated service short-circuit breaking capacity)  (Ics=25%Icu)

2 1
2

3 1
2
3

3 1
2
3

4 1
2
3
4
Icu  (Rated ultimate short-circuit breaking capacity)

2 1
2

3 1
2
3

3 1
2
3

4 1
2
3
4
* Sample For Indications
1 In of a given frame size.
2 This sample is omitted in the following cases:
A circuit-breaker having a single non-adjustable current setting for a given frame size;
A circuit-breaker provided only with a shunt release (i.e. without an integral over current release);
A circuit-breaker with electronic over current protection, of a given frame size, having an adjustable current rating by electronic means only (i.e. without change of current sensors).
3 Connections reversed.
4 Connections reversed, if terminals unmarked.
  • As Per UL 489, Paragraph 7.1.1.18: “if a circuit breaker is not marked “Line” and “Load,” one sample of each set tested, or one additional sample, shall be connected with the line and load connections reversed during the overload, endurance and interrupting tests”.
  • This UL test requirement specifies that for MCC to be UL Listed for reverse-feed applications, samples shall be tested with the line and load terminals reverse-fed, as shown in Figure, and that the test results shall be the same as those of “normally” fed circuit breakers. Depending on the design configuration and construction, the circuit breaker may or may not be affected by the application of power in a reverse-feed connection during these tests.

4

  • If Line / Load are not marked, we can connect Line or Load either on Top or bottom of MCB. However, it is a good practice to keep the fixed contact side connected to the bus bar.
  • For DC Circuit:
  • The Non polarized DC MCB have a No marking as ‘+’ and ‘–‘ symbol
  • Non polarized DC MCB operate safely as load breaking isolators and for fault current protection regardless of the direction of current flow through them.

5

Conclusion:

  • When a MCB are marked “Line” and “Load,” the power supply conductors must be connected to the marked “Line.” These MCB cannot be reverse-fed.
  • If “Line” and “Load” are not marked on MCB, the power supply conductors may be connected to either end. These devices are suitable for reverse-feed applications.

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(1) https://electricalnotes.wordpress.com/2016/10/04/how-to-select-mcb-mccb-part1/

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