MCCB: How to safely choose a new MCCB for replacement?

///MCCB: How to safely choose a new MCCB for replacement?

MCCB: How to safely choose a new MCCB for replacement?

The 3 phase Molded Case Circuit Breakers (mccb) used in commercial and industrial sites handle much more energy than the average household single phase miniature circuit breakers (mcb). It, therefore, poses a much greater risk during a fault if the component is not working properly.

Do not simply replace a Molded Case Circuit Breaker with one that has the same rated current. The most important consideration is the Ultimate short-circuit breaking capacity (Icu). Incorrectly replacing MCCB will result in dangerous consequences. In this video, we will guide you through the step by step process to do this.

5 Factors to Consider When Replacing an MCCB

Molded Case Circuit Breaker are trickier to replace as they typically handle more power than a Miniature Circuit Breaker. They are often found in the Main Switchboard or a Distribution Board

The important factors to consider are as follow:

  1. Frame Size
  2. Rated Current
  3. Ultimate Short-Circuit Breaking Capacity (Icu)

Other factors that may be important for your application are as follow:

  1. Operating short-circuit breaking capacity (Ics)
  2. Shun trip option

Example MCCB to be replaced

Schneider Compax NSX100B

  • NSX100 Frame Size (W x H x D) 140 x 161 x 86
  • Rated Current 100A
  • Breaking Capacity (Icu) (415V) 25kA

Frame Size

This determines the dimension of a circuit breaker. You will obviously have to select the MCCB with the frame size that can fit the space. The Frame Size indicates the basic framework of the Plastic shell of the MCCB that can hold the biggest rated current. It is the maximum current that a particular frame housing is designed for.

There are varieties current ratings MCCB for the same frame size. The following Schneider Electric Series has the same frame size:

  • Compact NSX 100
  • Compact NSX 160
  • Compact NSX 250

Rated Current

The rated current is the current value which overload protection is tripped and disconnect the circuit. You will obviously need to select the same rated current as the MCCB being replaced.

For an MCCB it is possible to have the option to adjust the Amp Trip (the current that the breaker trip) to that below the Amp Frame (The maximum overload protection current)

When replacing an existing MCCB, be sure to check this!

Ultimate Short-Circuit Breaking Capacity (Icu)

This can be defined as the maximum current from a short-circuit fault that can be safely cleared twice. This is tested according to the IEC 60947-2 standards. The MCCB will be reusable after interrupting a short-circuit fault, this is, however, only tested for 2 interruptions.

After interrupting short-circuit fault two times, it does not mean that the circuit breaker will fail to protect the circuit, it simply means that it has not been tested for.

It is also good to note that the higher the Icu for an MCCB is an indication of the higher MCCB quality.

The MCCB to be replaced is rated at 20kA for the 3-phase 415V. This is a typical value used in Singapore, however, please note that for different voltages the Icu values are different. Please ensure that you look at the Icu value for your particular operation.

In this case, we assume that the electricity supply is 415V AC

Why is the Icu so important?

If you choose the kA value that is lower than the actual Short-Circuit current then there will be dangerous consequences.

1 The contacts in the circuit breaker welded together preventing the circuit breaker from tripping. This will heat up the cable in the circuit.

  • The cable in the circuit is damaged breaking the circuit, or
  • A fire is started.

2 The copper in the circuit breaker overheating and turning into dangerous plasma.

  • The circuit breaker explodes

The consequences are very dangerous especially in the scenario 2 particularly if there are people nearby, for example, the electrician who comes to reset the circuit breaker.

*Make sure you select the MCCB replacement with the same or higher Icu value to the one being replaced*

The Shor-Circuit Current

You may wonder ‘What will the value of the short-circuit current be?‘. The short-circuit current value on the first circuit breaker depends on the size of the transformer and the length and size of the cable from the transformer to that Circuit Breaker.

The formula and the calculation of this are beyond the scope of this article. Here is a document from Bussman by Eaton explaining how to calculate the short-circuit current.

Guide to 'Short-Circuit Current Calculations' - Bussman by Eaton

The downstream circuit breakers short-circuit breaking capacity can be calculated from the distance from the first and the cable used. In reality, most designers will use a software to help to calculate the value.

In the case of a replacement, we will assume that the short-circuit current has been calculated and the initial MCCB designed has been done correctly. So we will only need to match the kA value of the breaker that has to be replaced.

Example replacements

LV429550 (original model)

Can replace? YES

  • Range name Compact NSX
  • Model NSX100B
  • Poles 3 Pole(s)
  • Rated current 100A
  • Ultimate short-circuit current breaking capacity (Icu)
    • (380-415Vac) 25 kA
  • Service short-circuit current breaking capacity (Ics)
    • (380-415Vac) 25 kA
  • Service short-circuit current breaking capacity as a percentage of Icu (Ics%)
    • (380-415Vac) 100 %
  • Dimension (WxHxD) 105x161x86 mm
  • Brand Schneider Electric

EZC250F3100

Can replace? NO!

  • Range name EasyPact EZC
  • Model EZC250F3100
  • Poles 3 Pole(s)
  • Rated current 100A
  • Ultimate short-circuit current breaking capacity (Icu)
    • (380-415Vac) 18 kA
  • Service short-circuit current breaking capacity (Ics)
    • (380-415Vac) 9 kA
  • Service short-circuit current breaking capacity as a percentage of Icu (Ics%)
    • (380-415Vac) 50 %
  • Dimension (WxHxD) 105x165x60 mm
  • Brand Schneider Electric

1SDA066807R1

Can replace? NO!

  • Range name Tmax XT
  • Model  XT1B 160 TMD 100-1000 3p F F
  • Poles 3 Pole(s)
  • Rated current 100A
  • Ultimate short-circuit current breaking capacity (Icu)
    • (380-415Vac) 18 kA
  • Service short-circuit current breaking capacity (Ics)
    • (380-415Vac) 18 kA
  • Service short-circuit current breaking capacity as a percentage of Icu (Ics%)
    • (380-415Vac) 100 %
  • Dimension (WxHxD) 76.2x130x70 mm
  • Brand ABB

1SDA067397R1

Can replace? YES!

  • Range name Tmax XT
  • Model  XT1C 160 TMD 100-1000 3p F F
  • Poles 3 Pole(s)
  • Rated current 100A
  • Ultimate short-circuit current breaking capacity (Icu)
    • (380-415Vac) 25 kA
  • Service short-circuit current breaking capacity (Ics)
    • (380-415Vac) 25 kA
  • Service short-circuit current breaking capacity as a percentage of Icu (Ics%)
    • (380-415Vac) 100 %
  • Dimension (WxHxD) 76.2x130x70 mm
  • Brand ABB

Operating short-circuit breaking capacity (Ics)

Ics is expressed as a percentage ratio of Icu and is the maximum short-circuit current that a circuit breaker can break three times. The manufacturers of the MCCB will test this according to the IEC 60947-2 standard. It is also a marker of quality as the higher the Ics, the more reliable the MCCB.

This is best explained with an example. The Molded Case Circuit Breaker has the following Icu & Ics value:

  • Icu = 40kA
  • Ics = 50% (i.e. 20kA)

Case 1: the Short-circuit current is at 15kA

This is below both the Icu and Ics and therefore we can assume that it can clear a fault 3 times as tested by the manufacturer.

Case 2: the short-circuit current is at 30kA

This is below the Icu value but higher than Ics value. This means that after the first fault there will be permanent damage to the circuit breaker. The MCCB can be reset to use again but is not tested to interrupt the short-circuit current again after the second interruption.

Case 3: the short-circuit current is at 45kA

The short-circuit current is above that of both Icu and Ics. In this case, the circuit breaker will fail as previously mentioned and the consequences are very dangerous particularly if there are people nearby, for example, the electrician who comes to reset the circuit breaker.

Shunt Trip from Protection Relays

A Shunt-Trip happens when the MCCB trip by receiving an external signal from a protection relay. Protection Relay is the name given to a group of products, which are sensors that detects a particular electrical fault and send a relay signal out. Example of these are as follow:

  • Over Voltage Protection Relay
  • Under Voltage Protection Relay
  • Phase Lost Protection Relay
  • Phase Sequence Protection Relay
  • Earth Leakage Protection Relay

You will need to check if the MCCB to be replaced is connected to any protection relays and purchase a replacement MCCB model that can accept the protection relay input and purchase the optional accessories for the particular protection relays.

We have outlined the steps that one should look at when replacing an MCCB from the frame size, the rated current, the ultimate short-circuit breaking capacity (Icu), the Operating short-circuit breaking capacity (Ics) and the shun trip option. We hope that we have stressed the importance of looking at the correct Icu value as the number 1 priority in the MCCB, or in fact any Circuit Breaker replacement.

In practice, one would try to have the exact model replacement, and when this is not possible, the take away is to look at the kA value carefully to avoid any dangerous consequences.

By |2018-05-04T16:16:47+00:00April 24th, 2018|MCCB - Molded Case Circuit Breakers, Power Distribution|