In some cases, it may be necessary to introduce extra rods to smooth out resistance readings, however, their corresponding depth of insertion can be reduced. This can be useful in situations where the soil hardness impedes the insertion of rods to their normal depths. The electrician must also ensure the meter(s) used to measure the required values of resistance, current and voltage in the circuit does not suffer any electrical noise distortion either due to the close proximity of the cables measuring the circuit or the presence of an external electrical network. These are technical nuances that untrained or unqualified personnel can miss when trying to determine whether the ground point being tested will safely dissipate static charges.
Another factor that must be taken into consideration when using fall of potential meters, is that they require high voltage inputs to break down the resistance of the soil. Extreme caution should be taken when readings are being taken in potentially flammable and combustible atmospheres.
From a technical standpoint, the fall of potential method offers a reliable means of estimating the contact resistance of the desired earthing point to true earth. The major drawback, however, is that it requires a qualified and experienced electrician to determine whether or not the static earthing point will function as intended. For processing sites that place heavy demands on their electrical and mechanical maintenance resources, delays to production, recovery and cleaning operations can occur if electricians are not available to measure and verify the earthing points to which the trucks must be connected.
Vehicle mounted static earthing verification system:
Responding to the demands of engineers and companies engaged in the transfer and recovery of flammable and combustible products, Newson Gale is conducting the last phase of “real-world” trials of a new truck mounted static earthing system that is capable of demonstrating full compliance with the API 2219 standard.
The Mobile Ground Verification system (MGV for short) has been designed in conjunction with hazardous material handling specialists, emergency responders and chemical manufacturing sites where limited access to electricians can delay hazardous material transfer operations.
The earthing system performs two primary functions that match the requirements outlined in the API standard. The first primary function is designed to automatically verify if the truck is connected to a ground point with a low enough resistance connection to true earth that is capable of dissipating static charges safely.
The second primary function verifies that the connection between the truck and the verified ground point is less than 10 ohms for the duration of the material transfer process.
User friendly interface and operation:
The MGV system has a user friendly operator interface which indicates when transfer operations are safe to begin. The earthing system is connected, with a quick release static earthing clamp, to either buried structural metal work or to rods that are hammered into the ground. When the operator connects the clamp to the object believed to have a low resistance ground connection, the MGV system immediately verifies whether or not the contact resistance of the object to earth is sufficiently low enough to dissipate static charges that could build up on the truck’s transfer system. This function is referred to as “Static Ground Verification”.
The system also confirms that the truck’s connection to the proven ground point is less than 10 ohms and monitors the health of this connection for the duration of the transfer process. This function is referred to as “Continuous Ground Loop Monitoring”. This function ensures that static charges generated on the transfer system have a clear, low resistance path to the verified ground point.
When both conditions are positive, a cluster of attention grabbing green LEDs pulse continuously, informing operators that they are in a safe position to start material transfer operations. A strobe light, which can be mounted at an elevated position on the truck, provides a wider field of vision to the operators when they are fully engaged in the transfer operation. Should the clamp be disconnected during transfer the system will shut down the LEDs and the elevated strobe light.
An additional factor of safety can be introduced by interlocking the pump with the earthing system so that if the truck loses its ground connection, the system shuts down the transfer process, thereby cancelling out the generation of electrostatic charges.
Simple visual diagnostic informs operator when a poor ground is detected or resistance in the ground loop is greater than 10 ohms:
If the system remains in a non-permissive state when the clamp is connected to the earthing point, a simple and effective visual diagnostic tells the operator if this condition is due to the ground point not having a sufficiently low enough contact resistance to true earth or if the truck’s connection to the ground point, via the earthing clamp, is greater than 10 ohms.
If the ground connection resistance to true earth is negative, the operator may reattach the clamp to another buried structure. If earthing rods are being used, the operator may spread a conductive solution around the rod, enhancing the conductivity of the soil, or relocate the earthing rod, or bury additional earthing rods to lower the system’s overall resistance to earth.
If the ground loop monitoring 10 ohm resistance reading is negative, the operator may need to ‘jig’ the clamp to enable the teeth to bite through contact resistance inhibitors, particularly if the earthing point is covered in protective coatings, rust or dirt. The system will then register if the clamp has achieved a connection resistance of less than 10 ohms.
This system is permanently fitted to the truck and is powered by the trucks’ 12V or 24V battery power supply. The system PCB is protected in a robust IP66 GRP enclosure and is supplied with an ATEX / Factory Mutual approved earthing clamp and a range of quick releasing Hytrel® protected 2 conductor cable options.
Mike O’Brien, Managing Director for Newson Gale
If you have any questions relating to the topics discussed in this article,
please contact Newson Gale.