A great deal of wisdom and experience go into writing the rules of the Canadian Electrical Code; however, the reasons may not always be clear to its users and sometimes we’re not completely satisfied to follow the rules without understanding the reasons behind them. This article reviews several rules from Section 10, Grounding and Bonding, and the reasons behind them.
Rule 10-700(2)(a) specifies that a manufactured grounding electrode may consist of at least two 3-metre ground rods driven full length into the earth, spaced at least 3 metres apart and bonded together. Obviously, achieving the lowest possible (or at least acceptable) grounding resistance is the primary objective of the rule, but why a minimum of 3 metres?
Starting from scratch, the measured resistance of any grounding electrode is the sum of all its resistances including the grounding conductors and connections, the contact resistance between the grounding electrode and the earth and the resistance of the earth. The first two are extremely small. We can, as a rule, neglect them, and it’s customary to consider the resistance of the grounding electrode to be the resistance of the earth.
To better understand this concept, we can think of the soil around each ground rod as a series of cylindrical shells spaced equal distances apart around the rod. The cylinder nearest to and around the ground rod will have the highest resistance since it has the smallest cross-sectional area and volume. As we move further away from the rod, each subsequent cylinder will have a progressively larger volume and therefore a lower resistance across it. In practice, taking a number of grounding resistance measurements at several distances from a single ground rod, we would find that approximately 25% of the total grounding resistance appears at .03 metres from the rod, 52% at .15 metre and 94% at 3 metres. In fact, we can only measure 100% of the total grounding resistance at an approximate distance of 7.6 metres to the rod.
Since most of the grounding resistance is nearest to the rod, we can without difficulty conclude that spacing ground rods closely together will not very much improve the overall grounding resistance provided by a single rod. When the rods are spaced closely together, overlapping current dissipation from the ground rods during a fault increases their voltages and the overall grounding resistance. As the rule prescribes, we need to install ground rods at least 3 metres apart so as to effectively reduce the overall resistance of our grounding electrode.
Rule 10-806(4) requires that magnetic materials (iron or steel) used to enclose grounding conductors must be bonded to the grounding conductors at each end. If a sleeve of iron or steel is used for mechanical protection, it amplifies the magnetic field around the conductor during current flow, increasing the voltage drop and impedance across the conductor.
How does bonding help? To reduce the inductive reactance due to the magnetic field, both ends of the sleeve must be bonded to the conductor so that the metal sleeve can carry a portion of the ground-fault current and to avoid an increase in the voltage drop and impedance in the conductor. This preventative measure is not required when using non-magnetic sleeves for mechanical protection.
According to Rule 10-700(3)(a) a field assembled grounding electrode may consist of a copper conductor at least 3 metres long, sized in accordance with Table 43, enclosed in the bottom 50 mm of a concrete foundation footing and at least 600 mm below grade.
How does enclosing a conductor in concrete provide an effective grounding electrode? Concrete located below grade has a somewhat lower resistivity than average loam soil. For this reason, in earth of average or high resistivity, encasement of a wire in concrete will result in lower resistance. This is due to the reduction of grounding resistance closest to the electrode. (From our earlier discussion, we already know that most of the overall grounding resistance will be found nearest the copper conductor.)
Rule 10-702 specifies that where there are multiple grounding systems for electrical, communications, community antenna and lightning protection systems, they must be separated by at least 2 metres from each other and bonded together by a minimum 6 AWG copper conductor. In the case of lightning protection, bonding between systems must be at or below grade.
What are the advantages of this rule? Separation and bonding together is required since a ground fault could occur on any of the systems and therefore to ensure a low impedance fault path, to clear faults on any of the systems as quickly as possible. Bonding between grounding systems is also required so that in the event of a lightning strike on any of the systems, damage may be avoided due to side flashes between the grounding systems.
If you are interested in finding out more on the background of any of the CEC rules, we will do our best to find out for you.
As with past articles, you should always consult the electrical inspection authority in each jurisdiction for a more accurate interpretation of any of the above.