How electrical selectivity helps installations live long and prosper

Selectivity is Scotty in the engine room rerouting the power

Selectivity isn’t a new idea. Everybody knows that when a fault occurs in an installation, a swift response is needed to limit the damage and keep the power supply to other parts of the plant running to ensure continuous operation.

What is selectivity?

Selectivity is insurance against a major fault, such as a compromise in a cable’s insulation or a situation that suddenly requires more energy than the circuits can withstand. The resulting overcurrent flows through all the circuit breakers from the power source to the fault point.

Selectivity equips the trip units inside each circuit breaker with the decision-making needed to correctly detect the short circuit, for example, by sending a ‘block’ signal to the breaker above them. The way this is done ultimately depends on what’s available to the operator: basic apparatus or advanced protection devices.

Selectivity techniques

Beyond this, ABB has developed a few advanced selectivity techniques that take protection to the next level:

  • Time-current selectivity – as the current increases, the trip time of the circuit breaker decreases
  • Current selectivity is based on the observation that the closer the fault point is to the power supply, the higher the current. Using this, you can set the instantaneous protections to different current values in order to discriminate the zone in which the fault occurs.
  • Time selectivity takes this idea further by also defining a trip time. A certain current value will make the protections trip after a defined time delay. The delay allows the protection placed closer to the fault to trip first, thus minimizing the exclusion area.
  • Zone selectivity is an evolution of time selectivity. Once the setting threshold has been reached, a dialogue among devices allows the fault zone to be identified precisely and to be cut out.
  • Energy selectivity exploits the current limiting characteristics of molded-case circuit breakers.

There are also automatic tools for building up selectivity – our electrical design software, DOC, is powerful and free to use – but selectivity is definitely something that has to be learned and ABB is here to help.

Selectivity is Scotty in the engine room, rerouting the power, “giving her all she’s got” to keep the Enterprise operational in an emergency. Without Scotty, you can only boldly go so far.

So why are there so many installations without selectivity on board?

One of the reasons is time pressure. Building a selective power protection system takes a little longer and making the electrical plant more flexible, durable and simpler to service just isn’t always part of the customer’s initial priorities.

At ABB, we believe quality means doing it right even when no one is looking.

And as the leader in low-voltage electrical solutions, it’s our mission to seek out new ways to make it simpler for others to do the same.

How can we support you?

ABB is committed to offering the best technical support possible. A great place to start is with our popular technical application paper ‘Low voltage selectivity with ABB circuit breakers’ or our new microsite dedicated to selectivity.

We can even offer personalized support with technical documentation for designers – just contact your local ABB sales office or for more complex networks, like microgrids and naval applications, contact our dedicated team at the Tech Center in Bergamo.

As somebody who has been there and done it, I know selectivity is worth the effort. Building an installation with selectivity may take a little longer –10-20% in my experience – but like Scotty once said: “A ship is only as good as the engineer taking care of her.”

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About the author

Alberto Bellini

I have been in ABB as electrical engineering consultant for over a decade. Now I am product manager of connectivity and system devices, Electrification Products division.
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