Whilst some may argue that mobile CCTV towers exist outside of the scope of BS.7671 (British Wiring Regulations) I firmly believe that all system designs and installations should adhere to these standards wherever possible. I see all too often equipment and deployments out on sites that are neither electrically nor mechanically safe and we have to ask ourselves how and why they are even leaving the workshop, let alone being deployed in situations where their failure in either respect could have serious consequences. Good craftsmanship aligns with the whole point of carrying out design risk assessments as to the suitability of a deployment in the first place and you simply cannot, or at least should not, be specifying or deploying equipment if it does not achieve even a basic standard, especially with regards to electrical safety.
The vast majority of equipment could be said to fall into the category of ELV >50dc but just because the voltage is reduced it doesn’t mean that if a fault occurs is any less problematic. A huge consequence of DC systems is that they run ‘hot’ so special attention should be paid to cable selection, terminations and the suitability of protective devices such as MCBs, where some brands are not rated for use in DC systems at all. Solar PV cables are a good example where the standard needs to be kept high, this is due to them carrying current for long periods of time especially during the summer months. It should also be noted that in any series linked arrays the combined PV voltage is nearly always greater than 50vdc prior to being rectified by an MPPT, so at this point the system technically falls outside of the ELV classification, why then do we see PV cables very often hanging free air or draped across the ground and usually having no form of mechanical protection. It’s a fortunate consequence that most systems we see require their PV cables to be fairly short so volt drop isn’t too much of an issue however if panels are parallel connected the current increases instead of the voltage, making correct cable selection even more important. The following image highlights just some of these problems and anybody operating equipment like this should be asking themselves ‘would you have it in your house?’
All cables and protective devices should be selected ideally in accordance with the tabulated values in BS 7671 or following the manufactures instructions. Battery cables in particular and subsequent links and fuse protection must be adequate due to the presence of high currents and their terminations must be suitably rated and fastened, paying particular attention to any specified torque requirements to avoid the risk of loose or undersized terminations leading to fires.
Devices for isolation should be suitably rated for the task, the image here for example shows an AC rotary isolator being used on (what appears) to be the PV incoming supply to the MPPT, this of course should be an isolator specifically designed to break DC current, where during switching without the correctly rated mechanism a ‘maintained arc’ could occur, leading to degradation of the switch itself and compromising its ability to do its job. It’s also worth noting that there appears to be no fusing present between the battery and the MPPT nor on the incoming supply cable, but then we should probably also ignore the fact that the cable itself is not DC rated in accordance with EN50618, the sheath eludes to AC @230v, its undersized and inadequately terminated.
Another issue that often arises is that in an electrical installation is there should ideally be a single point of isolation. This is sometimes difficult to achieve in a hybrid system where power will be flowing from several sources to run equipment. Measures should be taken as far as reasonably practical to limit the number of isolation points when they are used for emergency switching and should it be required to operate several switches this should be clearly labelled and also highlighted in the O and M manual.
The image above shows the main switch easily accessible and clearly marked along with an overall layout that allows for easy maintenance of the system components.
Containment of cables and some items of equipment should always be considered necessary, with any cable being adequately supported throughout its length. Certain situations may require a flexible cable be installed due to it needing to be moved or manipulated once installed, if this is the case proper selection of types HO7RN-F or 6181XY (BS.7889) should be used to allow for frequent movement.
Segregation of differing voltages should always be taken into account and where possible AC and DC should be installed separately. This is not always easily achievable when some manufactures supply their equipment with a mixture of AC and DC in the same enclosure with little or no ‘natural’ segregation, here all the installer can do is ensure that their cables are appropriately marked and then separated properly once they are within any containment. Signs should also be placed on containment or junction boxes that contain mixed voltages.
Where the classification of a unit can change dramatically is if it is mains connected at 230v. Many units produce 230v themselves via their own inverter equipment and in general a chassis ground is sufficient however once plugged in or hard wired they may be reliant on the supply cable to provide an effective earth. When designing and risk assessing a mobile towers use on a mains connected scenario you must take into consideration the characteristics and variety of the supply and confirm the suitability and effectiveness of the earthing. If a unit is to be supplied via a socket outlet this should be protected by a 30mA RCD at source but it is also important to ensure that the supply cable itself is of the correct type and size, with a minimum cross section of 2.5mm2 being required to meet regulations. If a unit is being hard wired for any period of time then it could be classed as fixed equipment and any appropriate electrical installation certification should be considered.
Poor installation as well as incorrect selection of batteries can lead to a fire risk, badly fastened terminations being a prime candidate for failure. Most battery manufactures will clearly state torque requirements for attaching battery leads and the vast majority supply the correct links and fastenings too. Correct fitting should always be ensured during assembly of a tower but the integrity of its connections should also be periodically checked throughout its service life as part of PPM. The very nature of this equipment being mobile increases the risk of something working loose while its being transported to site, so basic checks should always form part of any deployment before a tower is put into use, they should never just be dropped, turned on and left to it.
Flooded cells should only ever be used in enclosures if there is adequate ventilation and this needs carefully specifying for the amount of free air available. A huge problem with adding ventilation in environments such as Rail or Highways is that this affects the overall IP rating allowing the ingress of far more foreign objects, especially brake dust, so where a cabinet has to be ventilated this will potentially increase the frequency of PPM. The main issue of this is that there are more visits required to deployed units which in turn increases the risks and costs to the operator and end users respectively. The second issue with flooded cells is their gassing of of Hydrogen during charging and also the need for topping up electrolyte, the gassing being a problem without properly designed ventilation and again the topping up requiring further visits. Without paying proper attention to both these points there is an increased risk of fire so I would personally opt to design this battery type out of mobile towers altogether.
Lithium cells are increasingly used due to their ability to completely recover from deep discharge and there is an added gain in their reduced weight compared to Lead. Problems can arise in that typically their charge parameters are 5-30degC and it’s the upper limit that can become an issue if charging algorithms are not correctly set to prevent overcharging, especially in the confined space of a cabinet where heat can naturally build up over time. This increase of temperature should be taken into account and methods employed to reduce overheating as much as possible, non-insulated enclosures running PV equipment in particular can reach extremely high internal temperatures during the summer months even in the UK.
Sealed Lead Acid (AGM) cells overcome the main two points above but do bring into scrutiny manual handling operations. Any installation requires careful planning to ensure that should a cell need swapping the operation is a safe as possible. This is made more important if the works need to be carried out on site where an installer needs to ensure the terminations are back up to the standard mentioned earlier, where a poor or rushed termination again could lead to a fire occurring. If a tower is badly designed the whole operation of swapping a battery can take too long to safely achieve on site, especially on a highways deployment where installers only have a 15 min window to carry out a task without additional TM.
More and more were seeing fuel cells, especially Methanol being included in CCTV towers. Initially their use was as a form of slave backup to the PV, however now they are very often being used as duty. These fuel cells and their fuels bring about a whole new set of potential hazards and anybody involved in their installation, use and maintenance must ensure they are correctly fitted and operated. The picture below shows a blatant disregard for any sort of installer standards by the operator and whoever put it together in the first place.
Segregation and containment should ideally be the starting point for any fuel cell installation and arranging component parts in such a way that they cannot become a form of ignition is paramount. Consideration should be given as to if there is a spillage will it be contained to stop it polluting the environment. Dedicated bunded enclosures are the ideal scenario but the inclusion of bund trays can lead to manual handling problems with some of the larger cartridges available on the market. The image above has the complete system installed upon an open grid floor construction so if any spillage occurs it will run directly onto the ground but considering the fuel cell here is installed on a pile of polystyrene packaging they probably weren’t too worried about the environment unless it was an attempt at upcycling? If an accident were to occur in an installation as bad as this, the liability would probably sit with the operator who put it into service, but it should never have got that far surely? It might also be that the end user takes some responsibility for equipment installed on their site, if a client were to look in this cabinet I’d like to thing that they would reject its use.
Internal partitions or bulkheads can be used to effectively separate ignition sources from sites of potential leaks and this should be designed into any cabinets intended to carry fuel cells alongside electrical equipment. An added advantage of this is to control who has access to different items of equipment as a person not adequately trained in the use of Methanol or a fuel cell should not be able to integrate with one.
Methanol/air mixtures are hazardous to health as well as being potentially explosive with the concentration in air considered to be harmful to health (TWA 200ppm) very much lower than the one which is flammable. This requires that Methanol is only stored and operated in properly rated flasks and drums with appropriate seals and valves, under no circumstances should these be opened to atmosphere, as any contact with the fluid or vapour could have serious consequences. Correct warning signage should be used on any equipment incorporating a fuel cell to warn not only its corrosive effects but also the ability for its vapours to cause serious respiratory problems. Its safe handling is paramount and a risk assessment performed for every situation that requires its use, there is even an argument that the Fire Brigade should be made aware of any locations where it is in use.
Emergency procedures should be documented and appropriate training on the required response should be given to anybody operating a fuel cell but this should only be an addition to a properly designed and installed unit. The requirements for storage and transportation of the fuel itself must also be taken into account, with non-specialist vehicles not being able to carry more than 330l safely or legally, I wonder how many operators are aware of this? The safe use of Methanol as a fuel should overlap with proper disposal of any waste and the vast majority of containers are not refillable because of the hazards involved in opening a container.
Personally I’d like to see all mobile CCTV and Telecommunications equipment being required to achieve even just a basic standard of safety and this should start with their assembly and construction. If the same equipment were to be installed in a building it would have to comply with regulations so why not in towers? Currently there’s too many manufacturers and operators just ‘winging it’ and as I’ve written previously on the subject of Design Risk Assessment, this will only serve to drag down the perception and reputation of the industry. We all have an obligation to our own engineers as well as clients and the general public to manufacture and supply equipment that’s safe to use but it seems as though the industry is waiting for a high-profile accident to force any changes.
