This article looks at effective grounding and bonding, how it is defined by the code, and its importance to electrical safety.
The Canadian Electrical Code, Rule 10-500 Effective Grounding defines and specifies that: “The path to ground from circuits, equipment or conductor enclosures shall be permanent and continuous, and shall have ample ampacity to conduct safely any currents likely to be imposed on it, and shall have impedance sufficiently low to limit the voltage above ground, and to facilitate the operation of the overcurrent devices in the circuit.”
Appendix B offers some additional guidance by specifying that the impedance of the circuit fault return path must be sufficiently low so as to ensure that the overcurrent protection trips when it should to limit voltages on exposed metal and to permit minimum 5 x rated current to flow during a ground fault. “Rated current” is considered to be the rating of protection ahead in the circuit.
This article reviews several possible ground-fault return paths— the effective ones and the not so effective ones. When required, the Canadian Electrical Code requires that bonding conductors be installed in the same cable or raceways as circuit conductors. How important is that requirement and how critical is conductor spacing? The code does not specify any restrictions on the lengths of metal conduit or EMT when used as the equipment bonding means. Can metal raceways provide an equally effective fault path without any restrictions as to maximum length?
Let’s start with bonding conductor spacing. Tests have shown that bonding conductors must be close and parallel to the circuit conductors for the lowest impedance path for ground-fault currents. Inductive reactance increases with the distance between the circuit and bonding conductors and increases total impedance in the fault current return path. Inductive reactance always forces more current to flow along the paths closest to the circuit conductors.
Tests also show that the most effective fault return paths are bonding conductors inside cables or raceways, or metal conduit enclosing circuit conductors. Ineffective fault current paths are connections to grounded (or ungrounded) building steel, and bonding conductors external to raceways or cables. Also, due to its method of construction, metallic cable armour presents a higher impedance than metallic sheaths.
It can easily be demonstrated by testing that only an internal bonding conductor can divert a sizable amount of fault current from a metal raceway (approximately a 50-50 division). None of the other possible paths (building steel or external bonding) will have any appreciable effect on the direction of current flow.
The Canadian Electrical Code specifies no maximum length restrictions when metal raceway is used for equipment bonding. The cross-sectional area of a metal raceway is considered adequate to carry the available fault currents when selected in compliance with Tables 6 to 10. But is this enough?
The total circuit impedance of metal raceway consists of the raceway impedance, the couplings and the arc fault impedance. To determine the maximum length of metal conduit when used for equipment bonding, we must know:
- The minimum permissible ground-fault current (for effective grounding— at least 5 x the circuit overcurrent protection setting)
- The conduit impedance (from manufacturer’s data)
- The arc voltage (assume 50 volts)
- And allow a 50% margin of safety
Example – To calculate maximum ground fault impedance, for a 120/208-volt, 400-ampere circuit, assume a minimum fault level of 400 x 5 = 2000 amperes. Subtract the assumed arc voltage and divide by 2000 amperes.
120 – 50 volts = .035 ohms maximum
Next, find the conduit impedance at 2000 amperes per 100 feet (from wire and cable manufacturer’s data), apply a 50 percent safety factor and divide the permissible ground-fault return path (in this example .035 ohms) impedance by conduit impedance with safety factor to obtain the maximum permissible length of conduit. Although the Canadian Electrical Code does not specify it, maximum length is required by implication by Rule 10-500.
Based on the circuit protection rating, minimum bonding conductor sizes are selected from Table 16, but as a rule of thumb, they should have no less ampacity than 25 percent of overcurrent protection ratings. Bonding conductors must carry available fault currents during the clearing times of fuses or circuit breakers without any damage to wiring insulation, and without any risk of burning off.
Manufacturers can provide damage curve tables showing how much fault current and the lengths of time during which different size wiring can withstand faults without damage. Burn off tables are also available to show the melting points of copper and aluminum conductors at different fault currents and protection clearing times.
As mentioned above, distances between bonding and circuit conductors will affect bonding conductor impedance and resulting voltage drop during a ground fault. A higher impedance creates an increased risk of electric shock due to contact with exposed metal parts.
As in the past, you should check with the electrical inspection authority in each province or territory as applicable for a more precise interpretation of any of the above.