Transmission & Distribution Technology
Understanding the importance of cable cleats
Cable cleats should be regarded as a vital element of any cabling installation, says Richard Shaw.
Los tacos de cable deberían considerarse como un elemento importante en cualquier instalación de cableado, afirma Richard Shaw.
Kabelklemmen sind ein wesentliches Element jeder Verkabelung, sagt Richard Shaw.
L
aying electricity cables is time consuming and costly, and ensuring systems are properly installed is vital not just economically, but in terms of health and safety. Terefore, it would
seem extremely remiss, especially in an offshore environment, if one significant installation aspect was frequently neglected – cable cleats. Te importance of cable cleats is frequently
underestimated. What this means in practice is that instead of being treated as a vital element of any cabling installation, they are lumped in with the electrical sundries and seen as fair game for cost-cutting. But for an installation to be deemed safe, cables need to be restrained in a manner that can withstand the forces they generate, including those generated during a short circuit, and this is exactly what cable cleats are designed to do. Without cleats, the dangers are obvious – costly damage to cables and cable management systems, plus a risk to life posed by incorrectly or poorly restrained live cables. All of which is in stark contrast to the stringent safety requirements associated with offshore work. Unfortunately, it’s not just a question of installing any old cleat. It has to be correctly specified for the project in hand. If not, the cables might as well be secured with plastic cable ties. Te reason being that different cable cleats are designed to withstand specific forces, meaning the only thing underspecified cleats will do in a short circuit situation is add to the shrapnel. One reason for this worrying level of confusion is that the market is very much manufacturer driven. Terefore, the choice of product tends to be reliant on third party certification – in the form of a short circuit testing certificate – but unfortunately this can be misleading. For example, it is not uncommon for manufacturers to claim a given short circuit withstand at a given cleat spacing and legitimately provide third party certification to support this. However, the overlooked fact is that the quoted short circuit withstand is only valid for a cable diameter equal to or greater than the diameter of the cable used in the test. If the project in question uses smaller cables (and the
fault level and spacing is the same) then the force between the cables is proportionally greater and the certificate is inappropriate. Plainly and simply you cannot say that a specific cable cleat has a short-circuit withstand without qualifying the statement. So instead of claiming a withstand of 150kA you would need to say that the cleat has a short-circuit withstand of 150kA when securing 43mm cable in trefoil at 300mm centres. To me the only way of rectifying the issue
is through the adoption of cable cleats as short circuit protection devices. Te reason for this is simple. By giving cable cleats the same degree of importance as fuses or circuit breakers nobody would be left in any doubt about ensuring their correct specification. Te reasoning behind this is simple. In the
event of a fault, the forces between cables reach their peak in the first quarter cycle, which is the
Fig. 1. The UKL’s National Grid’s London Power Tunnels project.
www.engineerlive.com 13
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52