Wiring Up

Mains Power:

Although best left to those with the appropriate ticket (wiring licence) there are times that wiring the mains supply to a telemetry system may become the responsibility of the installer. This may raise fears in some and it must be stated from the outset that if you are not comfortable working with mains voltages then don't!

If 230 volts AC does not exactly give you the shudders then it is politely requested that you at least treat it with the respect it deserves. Unleashing a few kW, uncontrolled, can (and often does) lead to injury, often permanent. Still not frightened? Right, then this section is for you.

Telemetry systems usually require very little power and there is the tendency to use thin wire from the distribution cabinet to the sockets providing power to the modules etc. This is often disastrous as at some time someone will want to power a welding machine, a heater ("electric fire"), or even just a vacuum cleaner. All the items mentioned are capable of drawing well over 2kW at startup which translates into about 10 amps. This means the thin 5A capable wire is put through its paces probably leading to an accident waiting for a time to happen.

Two options are open to you. Either wire the socket with wire capable of handling the full current capability of the socket, or fuse or circuit breaker the sockets to avoid such currents being drawn.

If the first option is taken then using wire of 2.5mm² should prove adequate but 4mm² is more acceptable. Run lengths are also important when deciding on the wire size. If the sockets are to be available for heavy current drains then thought must be given to the loss on the cable from the distribution panel to the sockets. As a rough guide, the loss is approximately

V = 0.05 * A * L/mm²

e.g. the loss on a 20m of 2.5mm² cable run from distribution board to socket when powering an electric heater @ 10A is 0.05 * 10 * 20/2.5 = 4 volts.

If the second option is taken, this being to use a small core wire which is sufficient to supply the telemetry system and a soldering iron, then fusing or using appropriate circuit breakers is a must.

The only failing of this type of supply is the inability of the wire to handle fault currents that can be generated by the mains supply being many kA during a short circuit before the circuit breaker trips or fuse blows. During this time it is highly possible the wire itself could be damaged resulting is long downtimes and costly maintenance.

Thought must be given to the fact that faults are not only shorts but also surges in lightning strike areas. If the system is likely to suffer such surges it is highly recommended that suitable wire sizes be used. If a surge protection unit is installed ensure the wire ahead of the unit can accept any likely fault currents that could be found on the circuit.

A closing note: At some point have a full isolating breaker, one that breaks both Live and Neutral. If you have to work on the supply feed and the Neutral cannot be broken accidental shorting of Neutral and Ground, even though they are ultimately the same voltage, can result is very high currents.

Earth(s):

The earthing system has two major functions. The first, and most importantly, is to ensure all the equipment is kept at an equal potential, the second then being to ensure all extraneous energy resident on the system is drained to earth and therefore making it safe for humans.

Many will understand the second requirement, this being to ensure the equipment, when worked on, is at a safe potential. Being referenced to ground is the safest environment one could get. However, should the equipment be operating at some voltage above earth, even thousands of volts, it will still operate so long as all the equipment is at this potential.

The all important first requirement, of ensuring no potential difference exists between the various pieces of equipment, is often unknowingly overlooked. In an attempt to ensure all extraneous energy is drained, the installation could be found to have multiple earthing points - and this is a problem.

We deal with lightning protection later but suffice to say that every point where energy could be drained from the system to earth requires lightning protection. The more points there are, the more lightning protection is required. This is as a direct result of potential differences occurring during a surge should the energy drain unequally through the available earthing points.

The correct method of wiring an earthing system is based on a "star point" method, with the star point being the only point where energy is drained from the system. If small distances are involved (a couple of metres max) then the rule may be broken although if good practices are never compromised then errors never occur (ok, in theory anyway). Basing the earthing system on a star-point method does have an economic advantage as it reduces the amount of required lightning protection.

Many personally installed systems have not had any lightning protection whatsoever and have never been damaged, even with direct strikes. This was accomplished through ensuring the various pieces of an installation were securely bonded to each other (mainly with copper strap, not round cable) therefore limiting any potential difference between the various components. However, not installing protection is not recommended practice. Don't take the risk.

Lightning Protection:

Fitting lightning protection has one rule and one rule only - give the trapped energy a clear path to follow to earth.

Using this rule will ensure many years of unbroken service of the installations. Cloud to Cloud strikes usually have an effect on the antenna systems creating a large spike across the module's RF connector. Obviously this needs to be trapped and good low power surge arrestors are required to affect this trapping. There is usually little energy found on the mast itself and the screen on the coax is sufficient to drain this, via the equipment, to earth. Obviously a very weak ground connection could suffer under this condition.

However, ground strikes, even some distance away, can create large currents on the antenna installation and this energy will want to find a way to earth. The saying "it will find the shortest path" does not mean physical distance but electrical distance. If this happens to be the ground connection through your piece of equipment then this will be the path used.

The solution is to provide a path much shorter than the path through the equipment such that the energy is drained before reaching the ground connection of the equipment. This is done by ensuring the ground connection first reaches the protection before being taken on to the equipment. This also brings into question data line protection and all successful protection is based on a barrier method. All connections to the equipment are taken via a lighting protection panel such that should any one circuit have energy to drain all signals are raised to the same level. This avoids current flowing through the equipment.

There is a belief that 'floating' the installation protects it against such currents. Please believe this to be a fallacy. The strike has just travelled a few hundred to a few thousand feet down to earth. Is the last few feet above ground going to make any difference?

Signals / Data:

Wiring process signals into a radio telemetry module should preferably done via a marshalling panel that will allow testing of any or all signals with or without the module in place. Consideration should be given to analogue loops when the module is removed for changing or repair. During this time loops are broken and could therefore affect other local processes. Simple 'shorting bars' are useful for such times and well worth the time saved during de-installation and re-installation of the module.

Other advantages of the marshalling panel are testing points for measuring voltages and currents. Many times terminals for such testing are in obscure areas and pose a threat to the technician or engineer carrying out testing or repairs.

Facilities to break the loop in order to measure loop currents are also extremely important, if for nothing else than to have the usual screw terminals remain intact instead of stripped beyond recognition after being disconnected a thousand times!

Marshalling panels are of almost paramount importance if bared wires are used instead of the more professional approach of ferrules (some equipment have terminals so close ferrules cannot be used). Even after only a few dis and re-connections the wire needs to be made off again with the neat loom now looking extremely untidy.

Although the wire size used in a typical installation could be as small as 0.05mm² this is impractical. A personal favourite is 0.5mm², it is large enough to be handled easily but small enough to not prove too bulky.

In very large installations involving many I/O distribution point feeders and interconnections are preferred as ITC (indoor telephone cable) and Krone connector blocks. This serves as both a very convenient numbering system as well as offers all the advantages of the marshalling panel as described. As both legs of a loop are broken it offers a convenient point for testing loop leakage as described in a later section.

Although Krone blocks should only be pushed to accept 0.22mm² wire the fact that the wiring is only done once with all testing then done on the marshalling panel the reduced wire size proves not too inconvenient.