From the standards: Radio Frequency Sources (9kHz to 60GHz) for continuous transmissions and pulsed transmissions whose pulse duration exceed the thermal initiation time shall not exceed limits below:
State of legislation:
For Group I in South Africa, there were DMR directives (ME-2-2003 signed; MS-2-2010 unsigned) that contradict IEC/SANS60079-0 standard of 6W RF transmission stipulating 500mW.
I do not believe there is a mechanism to remove or update these directives? What happens if the requirements become outdated? Would it be worth including any DMR Directives in the next version or release of the ARP? That would keep requirements current & be clear to all.
The Mine, Health & Safety Act still specifically refers to ARP0108:2005? There have been several new releases of this document, with several new requirements inserted for mining. How do these new requirements in later versions of the ARP stand in law?
Is the 500mW limit in underground coal mines a limiting factor? What is the risk?
History: The 500mW limit dates back over 20 years & a lot has changed in this time. MS-2-2010 details concerns over spurious triggering of detonators & interference with electronic safety related equipment. It also states that testing to EMC standards SANS61000 (CE marking) will improve RF susceptibility.
Common sense though would suggest not putting the APs anywhere near the detonators anyway. In addition, RF fields drop away very quickly as you move away, so even at 1m, the RF field has reduced significantly. It does depend on antennae (directional, omni directional etc. and gain), but as an example using gain of 1:
Pr =Pt/(4 π d^2) : Power received = Power transmitted / 4 π x distance^2, so at 1m away from 100mW AP, the power is 8mW
Any susceptible collision detection systems should be reviewed & upgraded.
Section 3 of MS-2-2010 states use of ERP:
So the 500mW is not an absolute limit; a risk assessment should be done prior to use.
In conclusion, I do not really believe the 500mW is an issue any way. Typically, WIFI Access Points transmit at 100mW, so provided sensible gain antennae are used, there is no concern with respect to the 500mW limit anyway. See comment below on using LTE with Mobile Devices.
Rewards & benefits of wifi:
Wifi underground offers huge safety & operational improvements:
- Real Time Location Tracking Systems (RTLS): with the use of Intrinsically Safe wifi tags, everyone underground can be tracked for location. In the event of mustering or evacuation, the location of everyone is known & help can be sent to those that need it.
- Geo locations can be set up to prevent machines operating if there are unauthorised people present.
- Tags & mobile devices offer Man Down & Lone Worker functionality
- Communications via Intrinsically Safe mobile devices (smart phones, tablets).
- Access to documentation
- Training can be performed
- Remote Eyes & virtual remote support. Workers underground can be supported technically from an expert 1000s of km away without the need for travel & security etc. to get to the location of the problem.
- Data extraction can be used for operational analysis & optimisation.
Cell phones & SIM cards: Is there a risk if a SIM card is fitted to a mobile device?
Mobile devices typically transmit at <2W for GSM/3G/LTE & <<100mW for wifi.
So mobile devices could theoretically exceed the 500mW limit using GSM/3G/LTE, but these communications require communication with a tower & there is not one present in an underground mine!
It is highly unlikely that a mobile device will transmit without receiving a synchronization signal from a cell tower. If a device is connected to a network and you do go underground, the device will increase its power output trying to reach the tower it was connected to. But once that link is broken, it will not transmit until the sync signal is received.
Note though that if LTE is considered underground, the fact that mobile devices are likely to transmit over 500mW should be considered.
In conclusion, there is more risk by modifying the devices to prevent SIM cards being fitted (& invalidating the certification & warranty by tampering with an IS device) than the risk of LTE at 2W.
In the authors opinion, wifi and all it offers can dramatically improve safety in all industries including underground coal mines. In fact, the risks not using wifi is a far bigger issue.
There would be significant advantage to have legislation that clearly laid out what is allowed & what is considered safe.
Gary Friend PrEng
083 309 8200