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Improving cable connection reliability with EverLink™ for Schneider products

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A safer installation with NSXm distribution circuit breakers and GV4 motor circuit breakers

No matter what the application, correct power connections are critical to safety

One of the most important aspects of bringing power to any electrical installation is having proper connections. Even with the best of designs, if the electrical connections don’t hold up over time, serious problems will arise.

Whenever a connection is not properly tightened (or, equally, a conductor is damaged), a localized hot spot occurs: a possible consequence is the carbonization of the insulating materials in the vicinity of that conductor. This carbonization acts as a conductor enabling currents to run through them in the form of electric arcs.

What makes this seemingly simple part of the installation so prone to difficulty? It comes down to two things: ensuring proper connections at the time of installation, and ensuring those connections remain secure over time – which can be more challenging than one might think.

First, proper installation

Power equipment, such as circuit breakers or switch-disconnectors, frequently is designed with screw connections to allow a good, tight field connection. Such equipment always includes manufacturer specifications for the torque required to make solid connections. Operators know these specifications, but transgressions are not unheard of.

For example, when an installer is putting in a mixture of small equipment and large units, he or she might be tempted to use the same set of tools for both. In that case, it’s likely that the connections on the smaller equipment will be over-torqued or else, on the larger equipment, under-torqued.

Overtightening can introduce cracks, stripped threads, and lay the groundwork for future problems. Similarly, under-tightening increases the resistance to the electrical flow and opens the door for the cables to quickly oxidize and build up additional unanticipated resistance – which means they may soon begin to overheat.

Using the right tools

One common way to apply the correct torque is by using a calibrated torque wrench along with correct bits.

But the fact is, properly tightening the connection is only the first step. Yes, it helps prevent oxidation of the conductor, regardless of whether it is copper or aluminium, by starting out with as full contact between the contacts and the cable as possible. But over time, cables are subject to creep—they slowly reconfigure their cross-sections over time in reaction to the initial connection pressure. Ordinarily, that results in decreased connection pressure, which can lead to oxidation, which in turn increases the electrical resistance of the connection, which begins to build up heat.

The thermal effects of heating and cooling that occur as the current flow in the cable increases and decreases further accelerate the creep and the connection loosens.

And a hardware solution

To combat this inevitable loosening, Schneider Electric developed a creep compensating technology called EverLink, that has been recently incorporated not only into the new Compact NSXm circuit-breaker but also into the new GV4 motor starter. EverLink introduces a spring element into the connection that maintains the proper connection pressure for years despite creep in the cable and the ordinary thermal stresses created by current flow.

After the initial installation, for which proper torque has been confirmed using a breakaway bit (or another suitable method of confirming torque), the spring element ensures that no loosening of the connection occurs over time.

In November 2017, Schneider Electric’s test laboratories carried out comprehensive tests in which it compared three similar (160A) moulded case circuit breakers to measure the effects of connection unscrewing. The circuit breakers tested were Schneider Electric’s Compact NSXm, as well as circuit breakers from two leading competitors. The objective of the tests was to measure the level of heat generated on the terminal of each circuit breaker as a result of unscrewing the connection of each (thus emulating the cable creep and unscrewing effect caused by vibrations), at intervals of quarter turns up to a maximum of 1.25 turns. Each of the three circuit breakers was subjected to the same test in identical conditions. The results of the testing concluded that Schneider Electric’s Compact NSXm moulded case circuit breaker outperformed the competition. That’s to say, during testing, the Compact NSXm breaker generated the least heat of the three circuit breakers, by some margin, and as a result was the only one that did not trip after one complete screw turn. The competitors’ products no longer maintained sufficient pressure on the wire and therefore tripped when unscrewed by over a half turn.

Useful links

https://www.schneider-electric.com/en/product-range/63429-compact-nsxm/

https://www.schneider-electric.com/en/product-range-presentation/63428-tesys-gv4/

 

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