The Canadian Electrical Code has a long and precise definition for grounding as: "a permanent and continuous conducting path to the earth with sufficient ampacity to carry any fault current liable to be imposed upon it, and of sufficiently low impedance to limit the voltage rise above ground and to facilitate the operation of the protective devices in the circuit.” When we talk about grounding, we are usually thinking about electrical systems.
The code has an equally precise, but not as lengthy definition for bonding as: "a low impedance path obtained by permanently joining all non-current-carrying metal parts to assure electrical continuity and having the capacity to conduct safely any current likely to be imposed upon it. When we talk about bonding, we are usually thinking about metallic items such as equipment enclosures.
The rules for grounding and bonding are designed to protect people and property against the dangers of electrical shocks and electrically caused fires by:
- Limiting electrical circuit voltages
- Minimizing differences in potential between metal objects such as equipment enclosures and other grounded metal.
- Making sure that fuses and breakers trip promptly when they should
- Reducing voltage surges caused by lightning or other causes.
The code specifies that alternating current systems must be connected to ground for voltages up to 150 phase to ground or if the system carries a neutral.
The code also provides a number of exceptions where it would be more hazardous or impractical to operate with a grounded electrical system.
One such exception is electrical arc furnaces. It would be easy to see why a grounded system would be impracticable and maybe even hazardous, since the return currents might find many paths back to its source.
Another exception would be isolated circuits such as wiring to the underwater speakers in a swimming pool. If these circuits were to be grounded, under fault conditions, there could be a higher possibility of current flow in the water, which could be very hazardous to swimmers.
A third exception to the rule is an electric crane running across a hazardous area containing combustible fibres. It would be easy to see why it is advisable to eliminate some of the arcing and sparking risks so as to minimize the possibility of fire.
The code has some very specific requirements for grounding an electrical system. The system must be connected to ground once at the owner’s main substation or by the electrical utility and once again in the owner’s main service. The code prohibits any connections to ground once past the owner’s main electrical service.
This requirement has two main purposes:
- To maintain control of return currents by ensuring that they flow only on the insulated neutral conductor and not select random paths back to their source; and
- To ensure that the ground fault protection in switchboards will operate accurately and when it should.
The code is also very specific about bonding. It specifies that except for double insulated devices, metallic electrical equipment enclosures and raceways must be bonded to each other and to ground by a number of acceptable means such as bonding wires, busbars, metallic cable sheaths or metallic raceways.
The code prohibits using the identified circuit conductor (neutral) as the means of bonding electrical enclosures. Once again, there are several valid reasons:
- To maintain control of return currents as above; and
- To avoid grounding the electrical system past the main service, which the code prohibits; and
- To avoid the possibility that equipment enclosures could become live should the identified conductor (neutral) become accidentally disconnected.
The code goes to great lengths to ensure that all related metal objects remain at ground potential and do not develop differences in potential to each other. For these reasons, it specifies that in addition to the above, we must also bond to ground all of the following with a building:
- Metal water piping when not used for grounding the electrical system
- Metal gas piping
- Metal waste piping
- Metal supports for computer room floors
In dairy barns and other agricultural buildings where small differences in potential can have harmful effects upon animals (tingle voltages), all metal is bonded together, including all water piping, stanchions, water bowls, vacuum lines an even the floors where cattle are milked.
Another area where very small potential differences can be dangerous to people is that area in or around swimming pools. Section 68 of the code includes some very precise requirements for bonding all of the metal objects in the vicinity of a pool, in particular when electrical equipment is used within three metres of the pool.
As in previous articles, you should seek the advice of your local inspection authorities for an exact interpretation of any of the above as they apply in each province or territory.
Some Special Requirements
Several months ago in two previous articles, we covered some of the standard grounding and bond fundamentals contained in the Canadian Electrical Code. Now let’s look at some less usual or more interesting code requirements covering some special circumstances. It’s unlikely that all of our readers will have had an opportunity to employ all of the following.
One excellent example is Rule 18-132(2), which deals with bonding electrical equipment in Class I, Division 1 hazardous locations. Such locations contain dangerous concentrations of flammable gases or vapours which may ignite or explode on occurrence of a spark due to any cause including a loose bonding connection.
You may not be aware that Rule 18-132(2) has some exceptional bonding rules for electrical circuits, not only within the hazardous location, but also within the non-hazardous location from which the hazardous location circuit is supplied. (See Sketch A). This rule refers us to Rule 10-606(1)(a), (c) and (d), and Rule 10-60(2) (Means of Assuring Continuity at Service Equipment).
In simpler terms this combination of CEC rules tells us that circuit bonding in Class I, Division 1 hazardous locations and the rest of each circuit originating outside the hazardous location, must satisfy the special CEC conditions normally reserved for bonding on the supply side of customer main services.
As a direct result, electrical circuits in Class I, Division 1 hazardous locations and all the way back to their points of supply are restricted to the following methods of bonding and bonding connections:
- Bonding conductors
- Threaded couplings and bosses
- Grounding bushings and bonding jumpers
- Standard locknuts and bushings are insufficient and you will notice that EMT connections are not anywhere included in this list.
I have always interpreted the above rules as applying to only a complete circuit from its source (such as a distribution panelboard or motor control centre) in a non-hazardous location to electrical equipment in a hazardous location. But do these rules also affect the feeders upstream from these sources and perhaps back to the main substation? I hope someone can enlighten me. The code is not very specific on that point and I’m afraid you’ll have to discuss that issue with your local inspector for the best interpretation.
Livestock buildings and facilities also necessitate special grounding and bonding practices. CEC Rule 10-402(4) specifies that livestock waterers must be bonded with minimum #6 AWG copper conductors. Also, Rule 10-406(5) requires that all metal such as water pipes, vacuum lines, water bowls and stanchions must be bonded with #6AWG copper conductors.
For what reasons do we need these heavy wire sizes and special precautions? Tingle voltages (between electrical objects and the floor or between metal objects) in livestock buildings or other areas may originate from electrical ground faults in farm buildings or neutral potentials from primary electrical distribution systems. Cattle are extremely sensitive to very small voltages and will refuse to drink if tingle voltages are present in water bowls. They may also withhold milk during milking. Both of these actions along with their general unease in the presence of tingle voltages may lead to deterioration in health and a loss of production.
For these reasons and for safety reasons the CEC has created some special grounding and bonding requirements in livestock areas where these conditions may exist. To supplement the effectiveness of these rules, a device sometimes called tingle voltage filter is also available.
I am sure you already know that Rule 10-204 specifies that AC services, when required to be grounded, must only be grounded once at the customer’s main electrical service and once again back at the transformer supplying the service. Rule 10-204(1)(d) also prohibits any grounding or bonding connections to the system neutral downstream from the main service.
This sub-rule has two important purposes. It is designed to prevent grounding and bonding conductors, metal piping, ducts, cable sheaths or building structures from becoming parallel paths for unbalanced neutral currents. It also ensures that electrical system ground fault protection will operate effectively to eliminate dangerous and destructive arcing ground faults when called upon.
If that be the case, how do we treat an emergency standby generator which must have its neutral interconnected with the building system neutral? If the generator neutral is interconnected with its case inside the machine, there is no choice but to provide multi-pole switching to disconnect the generator neutral simultaneously with the phase conductors. If the neutral is isolated from the case and if not separately grounded, the neutral does not require disconnection from the electrical system neutral, but a separate grounding conductor must be installed to the generator case.
As with past articles, please consult your local electrical inspector in each province or territory for an exact interpretation of the above as applicable.
Read more by Leslie Stoch