Don’t Leave Grounding Mat Design to Chance – Protect Your Workers!
Pipeline designers frequently specify grounding mats to protect workers from step and touch voltages caused by AC induction. But often, they assume that the grounding mat adequately protects them from over-voltage conditions caused by lightning strikes. This can be a fatal mistake.
It is important to remember that induced AC current is not the same as current caused by a lightning strike. By comparing the waveforms of the two events, and examining the characteristics of grounding mats, the magnitude of the difference is evident. Thus, a ground mat design may be adequate to address the voltage drop created by induced AC current, but be completely insufficient in limiting the over-voltage conditions created by a lightning strike. For this reason, it is important to take both events into consideration when specifying a grounding mat to ensure worker safety.
The basics of gradient control mats
It’s helpful to review the basics of grounding mats – what they’re designed to do, how they perform their function, what their limitations are and how decouplers play a crucial role in this application.
The purpose of a gradient control mat is to limit step and touch potentials to safe levels for workers. It does so by limiting voltage radially away from a pipeline in the earth and to limit the voltage between the structure and the earth. Mats are metallic in construction and can be arranged in different ways around pipelines.
What is step potential? It is the voltage between a worker’s feet when standing on the soil surface. It’s based on a 1 meter separation between the worker’s feet, as specified by the NACE SP0177 standard.
What is touch potential? It’s the voltage experienced by a worker standing on the soil surface and touching the pipeline. This is based on a worker reach of 1 meter, according to NACE SP0177. Practically speaking, it’s the difference in voltage between any two points (any two metallic objects, for example). However this exposure is calculated differently and does not involve grounding mats or the soil.
AC faults and induced AC power frequencies are fairly easy to control with a reasonable ground mat design. Lightning is much more difficult to control. It has an extremely short duration – tens of microseconds – with a steep rise to peak voltage.
Often, many factors related to lightning are overlooked during grounding mat design and installation. This can expose workers to very large over-voltage conditions.
What type of grounding mat design is best?
The two most popular types of grounding mat designs are welded grid and spiral wire.
A single-conductor spiral mat looks just like its name implies. Seen from above, a spiral of zinc ribbon or wire surrounds a pipeline segment entering the earth. With the mat bonded to the pipe, AC current follows the spiral wire outward from the center. Notably, inductance increases with the distance away from the center.
In a grid-type mat, current flow occurs radially, moving in all directions away from the pipe to which it’s bonded. Inductance is much smaller than in a single-wire spiral mat design. It drops with radial distance away from the pipe.
What happens when lightning strikes these two types of grounding mats? Voltage limitation depends greatly on the design. With a single conductor spiral mat, it arcs from turn to turn. So it doesn’t provide much protection during a lightning strike.
A grid style map has much lower inductance. The resulting voltage immediately after a lightning strike is in the tens of volts, not kilovolts. Data based on lightning strikes shows that a grid-style mat maintains fairly low touch voltage. Moving outward from the center of the pipe to which the mat is mounted, the step voltage actually declines.
The difference in voltage ratios between spiral and grid style maps is literally thousands to one.
Wire length has a very strong effect on touch voltage. Increasing its length increases the inductance, which increases the voltage. Step voltage isn’t affected by wire length because you’re talking about the voltage differential between the worker’s feet.
The solution to this challenge is to use the shortest possible lead and preferably multiple leads to connect the mat. Using multiple leads cuts the inductance in half. That in turn reduces the current flow through the worker. In addition, if the mat is buried under 6-8 inches of stone, that further insulates him or her from AC fault voltage.
How the decoupler fits in
Regardless of which design is used, a decoupler should be placed in series between the pipeline and the ground mat, preventing corrosion issues that can result from this connection. With this, the cathodic protection will not be significantly impacted trying to protect the pipeline and the grounding mat. The decoupler will block CP current (DC) while simultaneously bonding the pipeline and grounding mat via AC continuity. With the decoupler in place, the grounding mat material has no effect upon the pipeline’s CP.
How can a decoupler be safely located between a pipeline and a grounding mat, as it would appear to the critical link between the two? Indeed, the decoupler becomes the sole bond, but is suitable for this task if certified and rated for the site conditions. Dairyland decouplers are independently certified by UL to meet various safety requirements, including AC fault and lightning current capability, hazardous location suitability, environmental exposure, and most importantly, the fail-safe design requirements, which is needed to assure safety grounding under all conditions – whether functional or not.
A decoupler and a gradient control mat make the best combination for employee safety and pipeline longevity in harsh conditions on the right-of-way.
Don’t leave the design of grounding mats to chance, which may expose your workers to electrocution hazards. Be sure to design and install grounding mats to provide adequate protection for personnel, both for step and touch potentials.
Regardless of the style of grounding mat used, a decoupler is a valuable addition to separate the grounding mat from the pipeline – to ensure cathodic protection while providing safety bonding and grounding.