You must start by differentiating the cable trays, each level of voltage has an assigned cable tray, each electrical field (and related fields) has a specific cable tray or a specific portion of cable tray. Each specific use or discipline has its own specific cables for technical reasons (interference, safety, etc.). There must be independent routings for each type of cable.
The different cable trays on a site are designed for:
- High voltage
- LV power circuits
- LV control circuits
- Control instrumentation (4-20mA, etc.)
- Low current instrumentation (thermocouples, etc.)
- Distribution bus instrumentation
- Telephony
- IT
- The earth of the equipment grounds (where applicable)
- etc.
All these cables and circuits are routed and cross over each other according to predetermined rules.
Therefore, when on site, please do not ask to add a cable to power your computer or control room TV by running your cable along a lightning conductor drop stack, for example, claiming that it is more convenient. (There is a good reason for this… ask your instructor for an explanation if you do not understand why).
You may get a few surprises: “why does the compressor cut out when we start the transfer pump?” If the vibration sensor cable or thermocouple cables pass close to the pump’s 6 kV cable, it is not surprising. (Other phenomena may very well not give any indication of what triggers the fault in the case of rapid transient induction.)
Cable trays are made of different materials (galvanised steel, stainless steel, fibre glass, PVC, etc.), different constructions (wire, ladder, perforated, etc.) and a wide range of colours, etc. Troughs, conduits and tubes (steel or PVC) are all similar to cable trays since they support/guide wires or cables.
In addition to providing mechanical protection, the covers of cable trays are mainly used to protect cables from the aggression of UV which damages the outer insulation sheath over time.
Laying Cables
The following tips concern aligning cables on cable trays.
Whether the cable trays are laid vertically or horizontally, a minimum installation distance must be maintained between the different types of cable trays. A distance of 200 mm is frequently used. However, check the correct specifications, as this may not be the case.
Cable trays are mechanically and electronically splinted together using a ground conductor connected to the general earth. This includes PVC and fibre glass cable trays (static electricity!)
Trefoil (clover leaf) or aligned configurations
This concerns power cables. The conductors of a cable carrying three phase current are manufactured twisted together to cancel (or rather to minimise) the electromagnetic induction produced by each phase (imagine the 3 vectors at 120°, their vector component is zero).
Three-phase cables (or three + N) may be laid either aligned or in a trefoil configuration. However, for high powers, when several single-pole cables form one phase, the trefoil technique must be used, the 3 conductors in the trefoil represent the 3 phases.
As a general rule, the cables must not be just “thrown” on the cable trays but aligned and attached. This is not just because it looks better but is also for maintenance purposes (it is easier to add/remove a cable) and to reduce the induction phenomena.
If you find cables which are heating up, or even a hot cable tray, this is not necessarily due to a current overload, it may simply be because the cables are incorrectly laid…
LADDER RACK
Ladder rack (also known as “cable ladder” or “ladder cable tray”)is a quick and easy method of transporting heavy-duty cables over long distances and in the worst site conditions since it can withstand high winds, heavy snow, sand or dust buildup, or high humidity.
The ladder is very strong and can be mounted in virtually any direction. Ladder rack is made of hot dipped galvanised steel. It can often be used in conjunction with a cable tray on an installation.
You can find ladder rack supports on site in switchgear/MCC electrical rooms and they are even very often found in the basement of those rooms.
THE DIFFERENT TYPES OF CABLE TRAYS
Types of cable trays available
- Ladder
- Solid bottom
- Trough
- Channel
- Wire mesh
- Single rail
Ladder cable trays
Ladder cable trays provide:
A solid side rail protection and good system strength with smooth radius fittings and a wide selection of materials and finishes.
- Maximum strength for long span applications with standard widths.
- Standard depths
- Standard lengths.
- Rung spacing.
Standard dimensions depend on the countries and the manufacturers.
Solid bottom cable trays
Solid bottom cable trays provide:
- Nonventilated continuous support for more fragile cables and an added protection for the cables made of metal or fibreglass.
- Solid metal bottom with solid metal covers for cables installed in overhead zones.
- Standard widths
- Standard depths
- Standard lengths
Standard dimensions depend on the countries and the manufacturers.
Solid bottom cable trays are generally used for electrical or telecommunication applications generating minimal heat and for computer applications with short to intermediate support spans. This type of tray is not recommended for use on sites due to the lack of ventilation afforded.
Trough cable trays
Trough cable trays
- Moderate ventilation with added cable support frequency and the configuration of the bottom part allows cable supports/tying at very short distances. Available in metal and nonmetal materials.
- Standard widths
- Standard depths
- Standard lengths
- Fixed rung spacing at the centre
Standard dimensions depend on the countries and the manufacturers. Trough cable trays are generally used for applications generating moderate heat with short to intermediate support spans of 1.5 to 3 m. The perforated cable tray is also a type of trough cable tray.
Channel cable trays
Channel cable trays provide:
- Cost-effective support for cable drops and branch cable runs from the backbone cable tray system.
- Standard widths in metal and nonmetal systems.
- Standard depths in metal systems and 1 depth in a nonmetal system.
- Standard lengths.
Standard dimensions depend on the countries and the manufacturers. Channel cable trays are used for installations with limited numbers of cable in the tray when conduit is undesirable. Support frequency with short to medium support spans of 1.5 to 3 m.
Wire mesh cable trays
Wire mesh cable trays provide:
A field-adaptable job site support system primarily for low voltage, telecommunications and fibre optic cables. These systems are normally made of zinc plated steel wire mesh.
- Standard widths
- Standard depths
- Standard lengths
Standard dimensions depend on the countries and the manufacturers. Wire mesh trays are generally used for telecommunications and fibre optic applications and are installed on short support spans of 1 to 2 m.
Single rail cable trays
These aluminium systems (other materials are now used) are the fastest systems to install and provide maximum freedom for the cable to enter and exit the system. Single-hung or wall mounted systems in single or multiple tiers.
- Standard widths
- Standard depths
- Standard lengths
Standard dimensions depend on the countries and the manufacturers. Single rail cable trays are generally used for low voltage and power cable installations where maximum cable freedom, side fill and fast installation are important factors.
Materials / finishes available for the various cable tray systems
Steel (Min. Yield = 33KSI) (35 KSI for stainless steel)
- Solid: hot rolled pickled and oiled steel as per ASTM A569 (Commercial Quality) or A570 (Structural Quality).
- Pregalvanised: galvanised milled steel as per ASTM A653 CS (Commercial) or SS (Structural Quality) G90
- Hot dip galvanised after fabrication: black steel which is hot dipped after fabrication as per ASTM A123.
- Stainless steel: type 304 or 316L fully annealed stainless steel.
Aluminium (Min. Yield = 23 KSI)
Alloy 6063-T6 or 5052-H32 as per ASTM B209.
Fibre-Reinforced Plastic (FRP)
Polyester and Vinyl Ester resin systems available.
Meets ASTM E-84 smoke density rating; Polyester 680, Vinyl Ester 1025. Class 1 flame rating and self-extinguishing requirements of ASTM D-635.
Cable tray mounting accessories
It is impossible to show all the items which could be used for the different types of cable trays, it would require a 200-page catalogue. Here are some examples.
When these accessories are unavailable, the following methods are recommended for the construction of various bends and junctions. It should be noted that all cuts in the metal should be painted with a rustproof paint.
How to construct a flat 90° bend for Cable Trays
How to construct a Tee piece
The first step is to mark and remove the tray lip (1). This measurement is equal to twice the width of the tray. We also remove one and a half times the width of the tray from the piece of tray to be added to the Tee.
The 2 pieces of tray are now bolted together (2).
Next we measure the gussets (3)
Once the gussets have been made, the final step is to bolt the tray together and produce a Tee piece (4).
How to Construct a 90o External Bend
This is perhaps the easiest bend to make. We first mark out the tray (a). The lines are drawn 75 mm apart, we cut down all 3 lines on both sides of the tray and bend to 90° as shown in figures (b) and (c).
To make bending the tray easier, try using a round object such as a scaffold tube, handrail, etc.
How to construct a 90° internal bend
We first mark and remove the tray lip (1). The lines are drawn 75 mm apart. Next we measure 12 mm on either side of each of these 3 lines, draw 2 diagonal lines from point A to B and point A to C on all the lines and on both edges of the tray and remove them. Then bend to 90° (2).
How to construct an offset in Cable Tray
First mark the tray (11), draw a centre line, measure 10 mm approx. on either side of the centre line and draw 2 lines. Now draw 2 diagonal lines from point A to point B and from point A to point C and remove as shown in figure (12).
Then bend the tray and measure the size of the required offset (13)
Once the size of the offset has been determined, mark out the cut on both edges, then bend to the required shape (14).
When the bends and offsets have been completed, all cuts must be painted with a rustproof/corrosion-proof paint. Those “operations” were carried out on a perforated galvanised steel cable tray, the procedure would be almost identical with a stainless steel or fibre cable tray.
Earthing the cable trays
Whatever the type of metal cable tray, it must be earthed:
- To the plant’s general earthing system every 15 to 20 m.
- To the plant’s general earthing system at its ends if the length is less than 15 m.
- All along the earthing wire connected with specific studs to ensure electrical continuity between the lengths. (as shown in the figure)
Also see the Company Standards on this point.