Selective Coordination with Molded Case Circuit Breakers

This is the fourth post in a series on achieving selective coordination – previous posts have provided an introduction to the topic, looked at its implementation in healthcare environments and considered some of the challenges of meeting related National Electrical Code® (NEC®) requirements. In this post, I’m focusing on molded-case circuit breakers (MCCBs), a common tool for overcurrent protection, and some of the principles designers and specifiers need to understand about how these devices operate in selectively coordinated systems.

MCCBs and their features

In the United States, MCCBs are listed to Underwriters Laboratories (R) (UL (R)) Standard 489, “Molded-Case Circuit Breakers, Molded-Case Switches, and Circuit-Breaker Enclosures.” This standard also applies to the smaller MCCBs often referred to as miniature circuit breakers (MCBs) and larger insulated-case circuit breakers (ICCBs). MCCBs have one or more instantaneous trip functions. Understanding what these functions are and how they work is critical, because the level of selective coordination achievable depends on whether or not a particular fault current will cause the circuit breaker to trip. These possible trip functions include:

• Fixed instantaneous trip function. This function is typically a feature of MCBs and small MCCBs. The amount of current these circuit breakers can experience without tripping instantaneously might be listed in manufacturers’ catalogs as the “hold” current, while the amount of current that will definitely cause the circuit breaker to trip might be listed as the “instantaneous trip” current. The difference between “hold” and “trip” currents defines the width of the time-current curve (TCC) band in the instantaneous range of the TCC.
• Adjustable instantaneous trip function. Larger thermal-magnetic MCCBs have instantaneous trip functions that can be adjusted in the field, with a typical range of five to 10 times the ampere rating of the circuit breaker. UL 489 allows a +30%/-20% tolerance on this function, which defines the width of the TCC band in the instantaneous region of the TCC. This tolerance is tighter on MCCBs and ICCBs with electronic trip units – typically +/- 10% – and it might have a wider adjustment range. If the trip unit is equipped with a short-time function (such as Schneider Electric’s MicrologicTM LSI and LSIG models), there often will be an “Off” position on the adjustment switch. Also, it’s important to note that manufacturers usually set the field-adjustable instantaneous trip function to its lowest setting in the factory.
• Instantaneous override trip function. MCCBs and ICCBs with electronic trip units typically also have an instantaneous override trip function that is factory set to protect the circuit breaker. This is important to note, because setting the adjustable instantaneous trip switch to OFF will not turn off the instantaneous override function. This function might appear on the manufacturer’s published TCC, or it might also be listed in a manufacturer-provided table. For any given circuit-breaker frame size, this trip level can vary based on the breaker’s interrupting rating.

Another important factor in how circuit breakers perform is their electrodynamic withstand – that is, the amount of fault current the current path can sustain before electrodynamic forces cause the contacts of the circuit breaker to blow open. This value determines the level at which the instantaneous trip function of the circuit breaker will be set, so it also has an impact on the degree of selective coordination that can be achieved in a given installation.

So achieving the highest level of selective coordination would require a circuit breaker to have high electrodynamic withstand, along with a trip system that can sense a high level of current – both of these are necessary. Additionally, an instantaneous trip function with a short time delay, perhaps a half cycle, would allow time for a downstream circuit breaker to clear the fault on its own.

Understanding TCCs – and their limitations

TCCs are a valuable tool for designing selectively coordinated systems, but they have some limitations and shouldn’t be the sole source of an engineer’s specification decisions. Both circuit breaker and fuse TCCs are developed by testing the device, itself, in isolation. This testing yields results that can be used to determine coordination in the overload region of the TCC of a circuit breaker – the inverse time part of the curve – and in the non-current limiting region of fuses (see Figure 1). However, this testing is inadequate to determine selective coordination in the instantaneous region of a the TCC of a circuit breaker, or in the current-limiting region of the TCC of a fuse.

Figure 1

The reason a TCC, alone, falls short for specifiers is that circuit breakers and fuses don’t work in isolation – they work in concert with downstream devices. Upstream circuit breakers and fuses open because of exposure to the peak current (Ip) let through by the downstream circuit breaker or the energy I2t let through by the downstream fuse. When the contacts of the downstream circuit breaker part or the fuse links of the downstream fuse melt, the resulting arc will limit the let through current and energy due to the dynamic impedance the arc introduces in the circuit. Unfortunately, TCCs tell us nothing about peak let through current or let through energy.

Fuse manufacturers solved this problem long ago by publishing ratio tables, and these tables must be used to determine selective coordination for fuses. Similarly, circuit breaker manufacturers publish short circuit selective coordination tables that, combined with TCCs, can be used to design selectively coordinated systems. The circuit breaker tables can be helpful when TCCs in the instantaneous region overlap at a point below the level of available downstream fault current. Manufacturers publish this data in varying forms, including:

• Tables showing the level of short circuit selective coordination between pairs of circuit breakers.
• Tables showing pairs of circuit breakers that will achieve overload and short circuit selective coordination in lighting panel applications.
• Tables that aid in the selection of transformer primary, secondary and branch circuit breakers yielding selective coordination and Code compliant transformer protection.
• Electronic tools that can be downloaded or used on-line to choose circuit breakers that will selectively coordinate.