Choosing Cable Cleats Over Ties for Critical Fault Protection

Budgets and timelines are tight. The last thing we should do is overbuild, but if the urge to simplify results in cutting corners, it exposes asset owners to unanticipated risk.

NEC Article 392.20 states that “single conductor cables shall be securely bound in circuit groups to prevent excessive movement due to fault current electromechanical forces.” It does not specify how this needs to be done, focusing instead on the outcome, which is cable restraint. This allows manufacturers to create innovative solutions to meet the requirements, but the upshot is that it creates room for interpretation by contractors which can result in inadequate safety measures.

Why Is Preventing “Excessive Movement” Important?

The amount of electromechanical force created in the first milliseconds of a short circuit event is astounding. When this happens, cables repel each other violently, and if not adequately restrained, will whip about the space causing physical damage.

In most cases circuits are not isolated, but packed close together on cable trays. A fault in a single circuit can quickly escalate into a multi-circuit event if unrestrained cables disrupt a neighboring circuit. Short circuit events are never planned, and replacing additional circuits can increase the length of an outage from hours to days, with significantly higher costs.

Ladder racks suffered extensive damage

For large enterprises, the average cost of downtime comes in at $540,000 per hour.

Source: Ketchum & Walton Co

Cable cleats are designed specifically to prevent excessive movement during a short circuit event. So, why would a contractor choose heavy duty cable ties instead of cleats?

A Real-World Example:

Panduit manufactures a complete range of cable cleat and cable management solutions. In addition to creating innovative products, our team also advises engineers on proper specifications for hundreds of projects every year.

In this case, cable cleats were specified in a project for a major company. The contractor installed cleats on the first phase but felt the process took a lot of time. To reduce labor costs, they proposed using double heavy duty plastic cable ties instead, fastened diagonally around the ladder rung to secure the cables. The company approved the change of specification, without understanding the ramifications.

To demonstrate the difference in performance between heavy duty plastic ties and the previously specified cable cleat solution, our team ran a full design experiment to assess compliance with NEC.

Double heavy-duty cable ties installed diagonally at every 2ft.

The Test

Two test rigs were built, each consisting of a 600mm (about 2ft) wide ladder rack, with 27mm (1.06 in) OD cables arranged in trefoil. Extra heavy-duty cable ties in a cross configuration were used at two-foot intervals to secure the cables. At the ends of the ladder tray, clamps were installed to restrict movement. The target fault level was established at 128kA, which the originally specified cable cleats passed without issue.

In Test #1, the load was set to just 50kA, yet all cable ties failed. The cable deflection was only limited by the width of the ladder, and any neighboring circuits would have been disturbed.

In Test #2, the rig reached only 104kA of the targeted 128kA because of significant damage. All cable fixings experienced complete failure, resulting in extensive damage to cables, ladder, tables, and surrounding straps and equipment. Over 2m/6ft of cable deflection was observed due to electromagnetic force, only limited by the length of the test rig.

Final Thoughts

Applying straps to cables at regular intervals is not the point of NEC 392.20. The goal is to restrain cables safely during an unplanned short circuit. Cable cleats will take longer to install than heavy duty ties, but if properly specified, they protect people and equipment for the life of the asset and minimize disruption should a fault occur.

Engineers can be confident that properly installed cable cleats meeting IEC 61914 requirements will do the job for which they are rated. Alternate strapping solutions can only be evaluated with testing, and as can be seen from the videos, getting it wrong can have consequences.

Don’t take shortcuts with short circuit protection. Panduit makes it easy to specify the right cleat for each project with our digital Cable Cleat kAlculator.

Our Technical Solutions Engineers are available for personalized recommendations to help you minimize labor and material cost, without compromising performance and safety when it counts. For more information visit, www.panduit.com/cablecleats.