Electrical safety

Leveraging selectivity to deliver overcurrent protection and critical load power availability

While safety through circuit protection is always the first priority, there are scenarios – especially in large, low-voltage installations – where total power shutdown while a localised problem is resolved would deny power to critical loads elsewhere. Learn how selectivity safely isolates faults but keeps critical loads up and running.

Author: Sebastian Oberwelland, Commercial & Industrial Buildings Segment Leader, Electrical Sector, EMEA

The basic premise of selectivity (previously known as ‘discrimination’) is simple. It is the coordination of overcurrent or residual current protection devices in serial connection, so that a fault in the installation is cleared by the protection device located immediately upstream of the fault. Overload protection devices such as fuses, different types of circuit breakers or residual current devices prevent unwanted tripping and` preserve system availability. The aim is to limit the number of loads that will be disconnected from the power supply if a fuse or breaker trips. So where and how best should selectivity be used?

International standards state that harnessing selectivity is a highly recommended, rather than mandatory, approach. And it’s one that offers particular benefits when it comes to electrical installations in marine applications and high-rise buildings, as well as systems where reliable power is crucial, such as those supplying medical devices, data centres or any other type of critical infrastructure. Further examples of the latter include life safety equipment such as sprinklers, smoke extractors, fire pumps and lifts that need to continue operating no matter the possible damage due to overcurrent.

This leaves system designers with a choice. They can take the route of merely complying with local and international standards – or exceeding them to achieve additional safety and reliability using a range of isolating electronic circuit protection methods.

What are the selectivity options?

Selectivity of two protective devices can be total, partial or none. Total selectivity is achieved where the overcurrent protection device on the load side will operate up to the maximum prospective short circuit current at its point of installation. In case of partial selectivity, the overcurrent protection device on the load side will operate up to the fault current, which is less than the maximum prospective short circuit current at its point of installation. This is expressed by selectivity limit current (Is).

While this division is accurate, from a practical point of view we’re ultimately interested in whether the combination of protective devices used in a given location is selective or not.

Various methods to ensure selectivity include:

Current selectivity
Time selectivity
Energy selectivity
Zone Selective Interlocking (ZSI)

Time-current selectivity takes both factors into account to solve selectivity under all expected circumstances.

Current selectivity involves different current values for instantaneous trip settings of protective devices in different parts of the installation. This is based on the fact that the closer a fault’s location to the power supply terminals, the greater the current will be. Inversely, the further a circuit protection device is from the power supply, the smaller the fault current. Each downstream circuit breaker on the load side needs to have a lower current setting than the next upstream device on the supply side.

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Time selectivity depends on differences in the response intervals between upstream and downstream devices, whereby the circuit breakers closer to the power source will take longer to trip than those on the load side. Time selectivity can be easily implemented by using two serial, non-current limiting circuit breakers with electronic trip units (MCCBs above 630 A or ACBs type). These enable a precisely adjustable time delay between the installation’s various protective layers.

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Energy selectivity involves the use of current-limiting circuit breakers, which are mainly MCBs or MCCBs up to 630 A. As the switching processes during disconnection of short circuit currents are very dynamic – within just a few milliseconds – it’s necessary to work with the let-through energy characteristics of both devices. Additionally, serially connected, current-limiting circuit breakers affect each other. When this occurs, it means that a simple comparison of let-through energies or time-current tripping characteristics will not provide reliable results. The only way to reliably determine energy selectivity is by referring to producer tables or software tools.

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ZSI is a more intelligent evolution of time selectivity that’s been developed to counteract possible time delay issues. Rather than tripping automatically if a certain current threshold or time delay is reached, circuit breakers equipped with the ZSI function can communicate with each other to precisely delimit and isolate the fault zone. This speeds up the time required to eliminate certain faults, without sacrificing the overall coordination or introducing the risk of nuisance tripping.

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Introducing Eaton’s NZM series with ZSI

Most of Eaton’s electronic trip units, such as those used in our air circuit breakers (IZMX/NRX) and moulded case circuit breakers (digital NZM series), offer this smart feature. Even better, communication between our devices with ZSI does not require an external power supply or any external modules.

The introduction of our latest NZM circuit breakers now brings further improved selectivity capabilities. These models – available from 2024 starting with NZM frame size 3, followed by frame size 2 – significantly improve overall system design. When a NZM3 breaker is used upstream with an NZM2 downstream, total selectivity is achieved, enabling smarter switchgear than ever before.

Eaton digital nzm pxr molded case circuit breaker models

To help you make the most of selectivity to strengthen protection and ensure your low-voltage critical load power supply, Eaton can help with all aspects of buildings electrical installation standards, design, technologies and circuit protection components. As well as publishing educational guides and selectivity tables for different types of circuit breakers, our support also extends to powerful time- and money-saving circuit breakers software. Our xSpider tool removes the need for costly studies or plotting complex time-current curves – making it even easier for you to maximise both system availability and protection.

Fundamentals of electrical safety in low-voltage installations guide

Learn more about low-voltage electrical installation design and circuit protection to IEC 60364 and IEC 61439-3 Low-voltage switchgear and control gear assemblies - Part 3: Distribution boards intended to be operated by ordinary persons (DBO) standards.

Download our free guide

Same footprint, more features, bigger opportunities

Eaton's new NZM moulded case circuit breaker (MCCB) series offers numerous features that maximize the safety, reliability and maintainability of modern energy distribution systems. These digital circuit breakers are also IOT-ready, thanks to integrated Class 1 energy metering and integrated communication.

Visit the NZM product page

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Leveraging selectivity to deliver overcurrent protection and critical load power availability

What are the selectivity options?

Current selectivity

Time selectivity

Energy selectivity

Zone Selective Interlocking (ZSI)

Introducing Eaton’s NZM series with ZSI

Fundamentals of electrical safety in low-voltage installations guide

Same footprint, more features, bigger opportunities

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