In order to dissipate static charges from the rail tanker, there is an expectation that the tracks on which the rail tanker is sitting has a direct connection to earth or is bonded to the loading gantry, thus equalising the potential difference between the gantry the filling arm) and the rail tanker. If this is the case the electrical continuity from the rail tanker back to the loading gantry, via the safety critical bonding connections between the track and the loading gantry, should be verified frequently, preferably prior to every transfer. Bond verification operations can be conducted by an electrician with a meter or the bonded connections can be verified automatically with a gantry mounted static grounding system. So instead of relying on a passive circuit to bond the rail tanker to the loading gantry, either method described above will ensure isolated rails on which the rail tanker may be resting and/or broken track-to-loading gantry bond connections are identified before loading commences.
However, there are many rail tanker loading sites in Europe where these assumptions cannot be taken for granted, especially when there are concerns regarding the track’s connection to earth. Some sites simply do not own the tracks that are present onsite which precludes engineers from conducting fall of potential tests on the tracks in order to determine if the tracks have a connection to earth. Because the site does not own the tracks their engineers are also limited by how much they can “interfere” with track, i.e. install their own bonding wires from the track back onto their property. Instead, the site will connect the rail tankers to the loading gantry with static grounding systems. The gantry itself should be earthed, hence any static present on the rail tanker will be dissipated to earth via the loading gantry.
Other sites across Europe choose to ground their rail tankers because the ground on which the network of tracks resist does not have a reliable connection to earth, hence they choose to ground the rail tankers with static grounding systems as a matter of good practice.
Industry codes of practice related to the static grounding of tank cars in HAZLOC atmospheres:
Working on the assumption that there is good electrical continuity from the tank to the wheels of rail tankers, the sections regarding rail tanker grounding in IEC 60079-32 and TRBS 2153 recommend a bonded connection between the rail tracks and the loading gantry not exceeding 1 meg-ohm. However, the vast majority of conductors that are used to connect rail tracks and to the loading gantry will be heavy duty metal conductors, –therefore a benchmark value of 1 meg-ohm seems unreliable as anything above 10 ohms would indicate a potential fault somewhere in the circuit.
North American associations like the National Fire Protection Association and the American Petroleum Institute each publish their own codes of practice for controlling the risks associated with rail tanker loading in EX/HAZLOC areas.
NFPA 77 “Recommended Practice on Static Electricity” (2014) and API RP 2003 “Protection Against Ignitions Arising Out of Static, Lightning, and Stray Currents” (2008) are publications written by committees of EX industry professionals who are recognised experts in the area of static control for hazardous areas. Where the grounding of rail tankers is referenced these publications do highlight the risk of non-conductive wear pads and bearings which could prevent static electricity passing from the tank to the wheels of the railcar, hence resulting in the hazardous accumulation of static electricity on the tank of the railcar.
What is clear from the recommendations of NFPA 77 and API RP 2003 is that 10 ohms in the grounding and bonding circuit is the maximum resistance recommended for equipment at risk of electrostatic charging in EX atmospheres. While API RP 2003 goes one step further in recommending 1 ohm or less, if a grounding system with ground status indicators is in use, 10 ohms is satisfactory. This is because the grounding system is continuously monitoring the resistance in the grounding circuit, so that if it rises above 10 ohms, the grounding system can signal this potential hazard to the operator of the loading gantry. Another important recommendation is to use interlocks wherever possible, to ensure the transfer does not take place if grounding is not present. By halting the movement of product, the charge generation source is eliminated thus preventing additional charging of the rail tanker.