Model GCM Gradient Control Mat by Dairyland Electrical

Personnel working near pipelines can be subject to electrical hazards such as lightning, AC fault current/voltage, and steady-state AC induction. "Ground mats" have typically been used at above ground pipeline structures where these hazards exist, but past designs do not provide adequate protection against lightning conditions.

DEI now offers an engineered Gradient Control Mat (GCM) to address lightning and AC fault current conditions, backed by analysis from experts in lightning protection and with a full review by Correng Consulting of the cathodic protection guidelines of this mat design. Step and touch potentials are now limited with the DEI grid-type mat, in a much lower cost design. DEI also recommends decoupling gradient control mats to improve CP on the pipeline, and offers the new, affordable Solid-State Decoupler (SSD) line for use in conjunction with the Gradient Control Mat (GCM).

• Low cost design.

• Galvanized steel grid, using 3" x 3" squares.

• Low inductance design limits step and touch potentials due to lightning.

• Decoupling the mat offers long life and improved CP on the pipeline.

• Data available on design life and supplemental anodes.

Detailed Description

The gradient control mat by DEI is designed to limit step and touch potentials due to lightning because such design will also keep step and touch potentials well below that required by IEEE 80 for power frequency voltages. The gradient control mat is made of 0.135” diameter steel wire with a 3” x 3” wire spacing with all wire intersections welded. After welding, the mat is hot dip galvanized. Mats are available in 4’ x 8’ sheets and are shipped flat on pallets. Each mat weighs 15 pounds (6.8 kg). For larger mats, these sheets can be joined together to form any size mat required. Provision is made for thermit welding adjacent mat sections together to form an electrically continuous mat and for thermit welding lead connections to the mat.

Design Life

To assure long life of the galvanized steel gradient control mat in all soil conditions, DEI has commissioned Correng Consulting to provide guidelines for cathodically protecting the mat using readily available magnesium anodes. The design life of the anodes protecting the mat is 20 years, as documented in Correng Report COR-05-9508D “Guidelines for Cathodic Protection of Galvanized Steel Gradient Control Mats.”

Installed Costs

Although auxiliary anodes and a decoupler are recommended, the total installed cost of the galvanized steel mat will normally be less than a pure zinc mat of any construction, particularly when an 8’ x 8’ or larger mat is required.

Safe Touch and Step Potentials

Specific technical data is provided for the touch and step potentials for the most difficult voltages to control, namely, voltage caused by lightning. Refer to Table 1 in DEI Application Note 8: Gradient Control Mats in Pipeline Applications for more information. To limit the touch potentials to safe levels, it is essential to make the connections between the pipe and the mat (whether directly or through a decoupler) with the shortest possible lead length, and preferably with multiple leads, to reduce lead inductance, and thereby reduce the inductive voltage drop in the leads.

Decoupling

While the DEI gradient control mat may be directly connected to the pipeline or decoupled, it is recommended that the mat be connected to the pipeline through a new, low cost Solid-State Decoupler (SSD) developed by DEI for this purpose as illustrated in Figure 1. A decoupled gradient control mat offers several distinct advantages:

• The galvanic potential of the mat material is irrelevant when it is decoupled from the cathodically protected pipeline.
• CP readings can be taken in the vicinity of the gradient control mat.
• Stray DC currents are prevented from accessing the pipeline, thereby preventing a corrosion problem where stray DC currents would exit the pipeline.
• Any interaction between the CP system and the gradient control mat functions are eliminated.
• When the mat is directly connected, the service life of the anodes is reduced by approximately 36% for the H1-Alloy magnesium anodes and 28% for the Hi-Potential magnesium anodes; hence, considerably longer anode life is achieved with a decoupled mat.