Brian Wilson, Managing Director of MineCom Australia Pty Ltd , presented this paper at a mining conference in Poland.
Redundant communications in coal mines
Two mine disasters in the USA and Mexico within months of each other resulting in the deaths of 77 coal miners have highlighted the inefficiencies of the communications systems currently installed in many coal mines in those countries.
2.01 In the Sago mine disaster, the miners thought that they were trapped with no way out. Due to the failure of the mines communications 'systems' the mines management was not able to communicate with the trapped miners. Had they been able to communicate with the miners, they could have given them directions for an escape route to safety.
However the mines communications systems as such, were destroyed in the initial explosion. Had there been some form of redundant communications installed in the mine, then the miners would be still alive today.
2.02 In the case of the Mexican mine disaster; the miner's bodies still had not been located weeks after the disaster occurred. Again the limited communications methods installed at the mine were taken out with the explosion, leaving management with no means of communicating with the miners.
As to whether they were alive and if so for how long, or dead, following the explosion was not known. As at this time no explanation has been given as to what caused the explosion.
3.0 Present day mine communications.
Communications systems presently found in coal mines in many countries, such as telephone systems, leaky feeder systems, fibre optic networks, 'through-the-earth' systems, all emanate from a safe area, usually located on the surface and terminate at or near the coal face dependant on the level of Intrinsic Safety approval. Should the cable be damaged/broken at any point after leaving the surface, ALL communications BELOW the break in the cable are lost.
Criticism of 'through the earth' communications systems, that have been promoted as a guaranteed form of communications under all conditions, (including the transmission of evacuation warnings to ALL staff) was raised at an MSHA public enquiry in Washington recently.
As it has not always been possible to install a single surface antenna loop at most mines, due to the mines location i.e. in or near a residential area, access rights or land ownership difficulties, the local terrain (mountains) etc requiring the use of an underground loop antenna. The use of an underground antenna is then no different to using a leaky feeder cable or telephone cable for communications; as the 'through-the earth' loop antenna would also be destroyed in an explosion/cave in underground.
Mine company representatives at the MSHA public meeting cited problems such as - shadow areas, or Nulls (no coverage) and interference problems, one mine actually only turned their 'through-the earth' system on, when they actually needed to send a message and promptly turned it off again immediately afterwards, due to severe interference with their mine telephone system. Had there been an accident, the cable would have been damaged or broken long before they were able to send out a warning to all miners, this would also prevent them from sending a message to any trapped miners advising them of any escape routes available to them.
Different dictionaries offer different interpretations of the word 'REDUNDANT' when associated with the field of Electronics i.e. -
• Duplication or repetition of elements in electronic equipment to provide alternative functional channels in case of failure
• A system design that duplicates components to provide alternatives in case one component fails
• Repetition of parts, or all of a message, to circumvent transmission errors
• Repetition of messages to reduce the probability of errors in transmission
4.1 Communications Backbone
In order for a communications system to be 'Fully Redundant', you need a communication backbone, which could comprise a combination of the following technologies -
• Fiber optic cable
• Leaky Feeder cable
• Copper telephone/data cable
• Wireless link (i.e. a 'wireless mesh' network)
4.2 Changes to the Mine Architecture
Whichever backbone technology you select, they MUST be capable of being interfaced together to provide a Continuous, Seamless, Communication Loop. In other words the backbone must travel underground and return to the start point.
While this many be a relatively easy task in a large factory complex or a open cut mine, it is not so easy in an underground mine situation. In order to provide a continuous loop of cable in a coal mine tunnel network, there needs to be more than a single entry/exit into the mine. To provide redundant communications for those miners working at or near the longwall or 'working face', an exit point needs to be located close by.
This exit point could be a bore hole, or a fresh air vent riser. In the event of an explosion in the lead up to the working face and the 'backbone' being cut or damaged, the miners at the face would still have a communications link back to the surface. Under present conditions, the communications link terminates at or near the working face, leaving miners with nothing in the event of an explosion.
The Sago mine rescue team provided vivid descriptions of the conditions and damage they found near the epicenter of the explosion. They described how they found steel girders, bent and twisted just like pretzels; anything manufactured from a lighter material was in fragments. From their description, it is doubtful that even an armored telephone cable, installed inside a heavy duty steel pipe and buried in the floor of the tunnel, would survive that type of explosion.
Therefore the only way to provide communications to those trapped underground would be to feed communications in from both ends of a cable 'RING', and provide a 'switch' similar to a 'Fiber Ring', that will switch the direction of communications should the cable be cut or damaged at some point.
5.0 Fully Redundant Leaky Feeder System
Fiber optic integrators offer a redundant 'FIBER RING' architecture (see figure 1 below), so that if the fiber ring is damaged or broken the data will still reach its destination by being diverted around
the cable break.
There is now a Leaky Feeder system on the market called SMARTReverse that offers a Fully Redundant solution similar to the 'Fiber Ring' architecture.
5.1 Transportation Tunnels.
This system was designed originally for increased safety in vehicle and railway tunnels; to ensure that communications were never lost in the event of a rock fall inside the tunnel e.g. communications between a train driver and the train control.
Each end of the tunnel has a dedicated link (fiber or wireless) into the railways communications network, ensuring that if there was a rock fall inside the tunnel, communications were still possible on either side of the rock fall. Under normal operation, communications back to the train control room would be via a fibre link or as in figure 2 below, via a radio link from one end of the tunnel, to the primary communications site.
SMARTReverse Train Tunnel Configuration, Operating normally - figure 2
In the event of a rock fall inside the tunnel the amplifiers on the 'downside' of the rock fall would reverse direction. These amplifiers would then communicate with the secondary communications site at the other end of the tunnel.
5.2 Mine Tunnels
To provide a similar level of safety in a mine tunnel situation, the most important criteria would be that there MUST BE TWO entry/exit points into the mine tunnel network. To provide maximum safety, the second entry/exit point must be located as close to the working face as possible.
The second entry/exit point could be any one of the following -
• a borehole drilled down from the surface enabling a leaky feeder cable or fiber optic cable to be interfaced into the main leaky feeder cable underground. The public enquiry in Washington (MSHA) was advised that a lot of time was wasted locating borehole drilling machines. A recommendation by the Sago mine rescue team that mines should consider drilling a series of bore holes in line with the main tunnels. A borehole located near the epicenter could be used by rescuers to drop miniature infrared cameras and/or microphones down and ascertain if anyone was pinned under rubble or injured and unable to move
• an 'airshaft', Escape way shaft or a winder shaft
• A safe alternate route within the existing mine tunnel network
As discussed earlier the fully redundant loop/ring can comprise of one or more technologies, see some examples below:-
• Leaky feeder cable covering the whole route
• leaky feeder underground and fibre optic cable on the surface linking back to the start point
• leaky feeder underground and a wireless link on the surface back to the start point
• leaky feeder and or wireless 'mesh' network underground and broadband cable back to the start point.
Reliable connectivity along the whole loop is vital for a fully redundant system to operate, battery backed power supplies & 24 hour access to any interface components of the system are mandatory.
It is advisable to have any interface (changes between 2 technologies) infrastructure located on the surface rather than underground. It is understood that the mine may not have ownership of all of the land above the underground tunnels, or the terrain may be too rugged to run cables, hence the need to use a second technology on the surface for the return path. In some larger mines it may be feasible and perfectly safe to use a secondary route back to the start point, without the need for a borehole to the surface. This is provided that a major explosion or rock fall in the main route into the mine, could not also take out the return path of the loop.
In the event of a break in the loop, the infrastructure used must be self healing i.e. the down side of the cable break must be capable of AUTOMATICALLY REVERSING the direction of the signals. This would mean in effect that the mine would have TWO independent communications systems operating, with the same level of coverage except for the damaged section of tunnel. The two 'legs' of the system would be linked at the start point, to provide seamless communications along both 'legs' of the system and to the outside world.
Traditionally, leaky feeder based in-line bi-directional amplifiers are designed to allow two-way communications up and down a leaky feeder cable. However, if the leaky feeder cable was damaged at some point, then ALL communications below the cable break (downlink) are lost. To provide increased safety underground, MineCom introduced its RINGFeeder leaky feeder system, that enabled the leaky feeder cable to be 'looped back' on itself using a secondary egress i.e. air shaft or bore hole, providing a continuous loop of leaky feeder cable. With RINGFeeder installed, if
the leaky feeder cable was damaged at some point, communications were still possible on the downlink side of the cable break using a designated operating channel, the miner only had to switch the radio to 'talk-around' mode.
Following on the success of MineCom's RINGFeeder leaky feeder system, which was designed for small mine operations, MineCom introduced SMARTReverse for larger mine operations. SMARTReverse is a fault tolerant leaky feeder system that can operate in the harsh mining environment and provide reliable communications on both sides of a 'cable break', on ALL radio channels, Voice, Data plus limited Video. With a SMARTReverse self-healing ring, if the 'ring' is broken, we electrically 'rotate' all of the amplifiers on the 'downlink' side of the cable break, so that they then operate in the reverse direction. Single pole double throw RF switches, with high isolation characteristics are utilised to 'electrically reverse' the normal uplink and downlink paths of the Amplifier.
The SMARTReverse Bi-directional amplifier actually contains 3 individual amplifiers providing amplification for Downlink - voice/data, Uplink - voice/data and Downlink - video. Normally the direction of bi-directional amplifiers is fixed, in the case of SMARTReverse amplifiers, the direction is controlled by the presence (or absence), of a 'control signal' which emanates from the SMARTReverse Controller located in the Head End on the surface. The 'control signal' is transmitted downlink using the Video amplifiers. A Battery backed DC power supply in the Head
End provides power down the leaky feeder cable to power the first few in-line amplifiers.
Over longer distances additional 'Power Couplers' are required to maintain the leaky feeder cable voltage back up to the recommended limits.
SMARTReverse amplifiers are equipped with 'drive-by' diagnostics in the form of ultra bright LED's indicating the condition of the amplifier. As an option, an on-board PC based diagnostics module can be added to the amplifier.
The diagnostic module measures a number of key parameters of the amplifier, i.e. leaky feeder voltage, current flow IN and current OUT of the amplifier, AGC voltages (signifying Signal Level) for the uplink, downlink and Video amplifiers. The PC based diagnostics module is interfaced to an RF data modem, linking the diagnostics module back to a PC located at the Head End, which can be linked into a PC network.
SMARTReverse is currently available for operation in the VHF 150 - 175Mhz & UHF 400 - 500Mhz frequency bands. SMARTReverse technology also requires a minimum of two entry/exit points to operate, additional entry/exit points would further increase/enhance safety in the mine, plus it can utilise one or more of the connectivity technologies discussed in section 6.
Under NORMAL operating conditions, the 'control signal' is transmitted from the Head End on the surface down the leaky feeder cable(s), holding all of the amplifiers in the 'FORWARD' (normal) direction.
Should the leaky feeder cable be DAMAGED at mid-point along the tunnel, the 'control signal' would NOT be received by amplifier A3 and A4. Amplifiers A3 and A4 would then reverse direction automatically.
Once the damaged cable section has been repaired or replaced, amplifiers A3 and A4 would receive/detect the 'control signal' again and revert back to their normal 'FORWARD' operating direction, the system would then be operating normally.
There are some points that MUST be taken into consideration if the leaky feeder system is to work reliably and be available when it is most needed, in an emergency -
• produce a scope of works that defines your mines communications requirements - Today, 12 months down the track, and even 2, 3, 4 and 5 years down the track
• do not be satisfied with "yes our system does that function", ask where you can go and see a system working in the field with those features operational, or at least talk to a mine that has that brand of system installed.
• a leaky feeder system MUST be designed around the mine tunnel network it is to be installed in. One design DOES NOT suit all.
• amplifiers MUST have ALC (Automatic Level Control) or AGC (Automatic Gain Control) LLC (Line Length Compensation)
• plan for the installation of the leaky feeder system well in advance, - Head End location, power requirements, PABX connections, fibre connections, concrete foundations if required etc. The leaky feeder cable should arrive well ahead of the other hardware as it takes longer to install(most mines install the leaky feeder cable themselves and simply get the supplier to install the remaining hardware and commission the system)
• have at least 2 or 3 mine electricians trained by the manufacturer/supplier for maintenance of the system and to ensure timely expansions of the system. (every time an electrician leaves, organise training for their replacement)
• you MUST maintain the system and you MUST carry out preventive maintenance of the system if it is to provide reliable service
• Amplifiers MUST be installed to the manufacturers recommended spacing and no more, placing them too close together can cause crosstalk problems. Some newer amplifiers on the market will adjust their output levels automatically (including SMARTReverse) to match the required spacing/ distance to the next amplifier.