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Circuit Protection

What do all the different terms mean when discussing circuit breakers and how are NEMA and IEC terms different? by jraef
Posted: 4 Dec 07 (Edited 7 Dec 07)

In the world of Circuit Protection Devices, people tend to use a lot of abbreviations and acronyms that sometimes leave readers with confusion because they may be different from what they have been taught or picked up along the way. This FAQ will list the common terms ands provide brief descriptions to help untangle some of the confusion. This is intended to be a work in progress, please feel free to make suggestions for additions or deletions. I am using two terms that need a little definition: "NEMA world" refers to North America, i.e. USA and Canada (and to a lesser extent, Mexico) where standards are developed by NEMA (National Electrical Manufacturers Association) and acceptance testing is done by UL. "IEC world" basically refers to most of the rest of the world that adheres to the standards put forth by the International Electotechnical Commission, who's manufacturers generally self-regulate and test. I will start off randomly and eventually reorganize it in alphabetic order.

MCB In NEMA world, this used to exclusively mean "Main Circuit Breaker", meaning the primary circuit breaker in a group or panelboard / switchboard. In IEC world, it means "Miniature Circuit Breaker", typically a DIN rail mounted circuit breaker in a small format that is limited to 100A, and is intended for use in installations with low fault current capacity (under 25kA) unless protected by upstream fuses. In NEMA world, these devices entered under special rules adopted by UL (UL1077) and are called "Supplementary Circuit Protectors" because they did not meet the criteria already established for "circuit breakers" under UL489. In recent years, many manufacturers have come out with special versions that do now meet the UL489 standards for circuit breakers, so the term MCB has taken on a dual meaning. So in NEMA world, the acronym MCB now has contextual relevance. MCBs / Supplementary Circuit Protectors are sealed units that can be neither maintained or repaired, they are throw-away devices.

MCCB Was originally an IEC term meaning Molded Case Circuit Breaker, but has been very well adopted in NEMA world now, at least at the engineering level. Many rank and file electricians still are not familiar with that acronym because its widespread use in NEMA world is fairly recent. MCCBs generally range from 15A to 1600A, with a few exceptions at either end, and generally start at 14kAIC but go up to 100kAIC for applications with high fault current capacity. There are versions that are Current Limiting (CL) as well. MCCBs are typically sealed units and cannot be serviced, although in larger sizes there are versions with interchangeable parts that can be replaced such as trip units and switch bodies.

MCP A NEMA world acronym for Motor Circuit Protector, specifically describing a magnetic-only MCCB (no thermal trip elements) that is to be used STRICTLY as part of a factory assembled, listed and tested motor starter assembly. They are often mistakenly specified and used in user-built assemblies, but this is not acceptable in the NEC (US) and CEC (Canada). In IEC world, a Motor Circuit Protector can sometimes mean a completely different device, although generally the MCP acronym is not used to avoid confusion. Devices that incorporate Short Circuit (magnetic instantaneous) trips, adjustable thermal (Overload) trips and a disconnecting means, specifically designed for motor loads, is called a Motor Circuit Protector by some manufacturers, a Motor Protective Switch (MPS) by others, a Motor Starter Protector (MSP) by others etc. etc.

ACB Short for Air Circuit Breaker, the IEC term for what is called an Insulated Case Circuit Breaker (ICCB) in NEMA world. Either term is describing a large low-voltage circuit breaker designed for high current applications up to about 6300A. ACBs typically have the facility to adjust or even disable the instantaneous trip in order to maintain protection coordination with downstream devices. The breaker is designed to be mechanically very robust in order to resist the powerful mechanical forces which exerted during fault conditions, wile waiting as long as possible for coordinated downstream devices to trip first. So essentially, an ACB/ICCB is used as almost the last line of defense during a fault. ACB/ICCBs are designed to be field maintainable, unlike MCCBs which are not.

Side note: I have seen several websites recently, particularly from China, where they use the term ACB when describing MCCBs, which is something to watch out for. This is probably a translation error on their part but is potentially dangerous if you buy an ACB from them and receive an MCCB because MCCBs are NOT designed to withstand the same kind of mechanical forces.

PCB Is short for Power Circuit Breaker, a more generic term frequently used in the past as somewhat interchangeable with ICCB when discussing Low Voltage systems, but also means large breakers in Medium Voltage systems. This was because you could have a LV Power Breaker that was either an ICCB or an ABCB, a version that used compressed air to blow out the arc when opening the breaker. ABCBs have now become the exception to the rule as ICCBs have become a lot less expensive and more reliable, so there is probably no longer a need for a generic term in LV systems. So more recently there has been concerted effort to move away from using PCB to describe Low Voltage breakers in order to help avoid confusion. But even then, the term PCB is being thrown around now as a generic Main Circuit Breaker for everything from residential load centers to electronic power supplies in appliances. So for the most part, Medium and High Voltage systems engineers are now using more specific terms depending on the design of the Power Circuit Breaker in question, i.e. Vacuum Circuit Breaker (VCB), Oil (Insulated) Circuit Breaker (OCB or OICB) or any number types of Gas Insulated Circuit Breakers (no acronym that I am aware of) such as SF6, sometimes called "Puffer Circuit Breakers".

Thanks to members ScottyUK and dpc for their collaborative efforts.

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