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Protecting personnel and equipment in everyday operations:
Most chemical processing sites have designated grounding points to which trucks must be connected prior to transferring product. Normally, these sites follow a strict policy of adhering to the standards whereby electricians regularly check the resistance of the grounding points prior to every transfer process ensuring they have not degraded because of environmental and industrial life-cycle effects.
In hazardous material recovery operations, for example sucking up spillage from road & rail incidents, designated grounding sources are not available to ground the truck. In such events, the recovery team must bury rods in the ground and take resistance readings of the rods to ensure they have a secure connection to true earth.
For retail petroleum and gas distribution and the food and beverage industry, many point-of-delivery destinations do not have static earthing systems installed and the best the transporter can expect is that there is a designated earthing point to which he can connect the delivery truck. However, he will have no means of confirming whether or not the grounding point has a low resistance connection to earth.
Based on what the API standard recommends the two key conditions for protecting personnel and equipment from incendive static spark discharges depends on:
- Verifying that the resistance value of the ground point (the ground source) to true earth is of a known and measured value that is capable of dissipating electrostatic charges.
- Ensuring the connection resistance between the truck and this verified grounding point is less than 10 ohms.
Fall of Potential 3 Point Test Method
Verifying the connection resistance of an object to true earth:
The most common method of measuring the resistance of objects located in the ground to true earth is the “fall of potential” 3 point test method. This method is designed to measure the contact resistance of the surface of the electrode to the soil it is in contact with. Soil resistivity levels can vary greatly ranging from very low values of around 2 ohms for marshy ground to over 1000 ohms for rocky ground.
The 3 point test method is a relatively complicated and time-consuming procedure requiring a certified electrician, with the requisite level of experience and training, to follow the correct procedure and determine when a reliable, low margin-of-error, resistance reading is established.
Essentially, 3 electrodes, including the one to be tested, are inserted into the ground and a range of readings are taken until uniform and consistent values of resistance are obtained. Skilled interpretation and interpolation of the acquired data is required to ensure that the readings are not taken at “face value”.
To illustrate, there are specific depths to which rods should be buried in relation to their respective distances from each other and the electrician must know where to position the rods to ensure the soil resistance between the electrodes do not overlap and distort the readings. The rods must also be buried in a straight line relative to each other.
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 grounding point to true earth. The major drawback, however, is that it requires a qualified and experienced electrician to determine whether or not the static grounding 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 grounding points to which the trucks must be connected.
Contact us on the phone number / e-mail address provided on Page 3, to learn more about how the MGV system is operated.
Vehicle mounted static grounding 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 grounding 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 grounding 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.
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