The CE Code requirements for bonding and grounding are perhaps, the most important safety rules for electrical installations. Each person deemed to be qualified to do electrical work must clearly understand what bonding and grounding is, why it is necessary and how it must be done. And yet the basic principles of these essential code requirements are often misunderstood.
Sometimes the Code users are confused by the terminology of Rules that govern this important subject.
For example, let’s look at the terminology which is used in a single Code Rule that mandates grounding connections for AC systems. Rule 10-204 covers criteria for grounding connections and by specifying this criteria uses such terms as grounding circuit in Subrule (1)(b); grounded circuit conductor in Subrules (1)(d) and (2); grounded conductor in Subrule (3) and system grounded conductors in Subrule (4).
Rule 10-206 (1)(b) introduces another term a common system grounding conductor, and Rule 10-624 offers another interesting variation by applying term grounded service conductor.
Are all these terms really different, and if they are, what is the significance of their use? Let’s review a portion of installation between a secondary of a distribution transformer provided by a utility (or by a power supply authority) and a consumer’s service box in a building. The secondary side of this utility transformer represents a start of a new electrical system which supplies that consumer’s service. If a neutral point of such system is connected directly to a grounding electrode (via a grounding conductor), then this is a solidly grounded system. If this solidly grounded system originates from the transformer secondary that has only one winding with a middle (neutral) point being solidly grounded, then it is a traditional 1-phase, 3-wire system (usually 120/240 V). All Code users are well aware that the voltage between the grounded middle point (neutral) and each end of the winding is 120 V to ground. The voltage between two “hot” ends of this winding is obviously 240 V (line voltage).
If the windings of the transformer secondary are connected together in a “Y” configuration, and the common point (neutral) of this connection is attached to a grounding electrode (also via a grounding conductor), then the system originated at the transformer secondary is a 3-phase 4-wire solidly grounded system.
Let’s look at the Code terminology through a few diagrams of service connections.
Figure 1 illustrates a typical single-phase 3-wire solidly grounded system (120/240 V) that originates at the secondary of the power supply transformer. A grounding connection is made at this transformer in accordance with Rule 10-204(1)(c) of the CEC . Service conductors that run to the service box consist of two ungrounded (“hot”) conductors and one grounded (neutral) conductor.
Neutral conductor must be sized in conformance with Rule 4-022 of the CE Code As explained in Rule 10-204(3)(c) of the Code.
This figure shows that a connection to a grounding electrode is also made at the service box [Rule 10-204(1)(a) and (b)]. It is interesting to note that bonding of the service box is permitted by use of the grounded service conductor (neutral) on the supply side of the service disconnecting means. Neutral conductor is not permitted to be used for bonding purposes of any electrical equipment on the load side of the connection to ground. A separate bonding means conforming to Rule 10-804 of the Code must be provided for equipment bonding. On the load side of the service disconnecting means the neutral only serves as a grounded circuit conductor, and in a typical 2-wire circuit it carries the same current as the ungrounded (“hot”) circuit conductor.
Figure 1. Grounding connections of single-phase 3-wire solidly grounded system
Quite often a solidly grounded system does not use a grounded service conductor to carry a full load or unbalanced current. In this case, the grounded conductor is not used as a grounded circuit conductor (as a neutral conductor). It is used only for metering purposes, and it is terminated at the service box and in the meter mounting device.
“Figure 2. Grounding connections of three-phase 4-wire solidly grounded system without a grounded circuit conductor (neutral) on the lead side of the service disconnecting means
Figure 2 shows that although a 3-phase 4-wire solidly grounded system is originated at the secondary of the power supply transformer, the grounded service conductor run to the service is not used as a neutral (i.e., is not intended to carry a load or unbalanced current). Thus, this grounded conductor must be sized in accordance with Table 16 [re: Rule 10-204(3)(b)]. All grounding connections at the transformer and at the service equipment must meet requirements of Rule 10-204 for solidly grounded systems.
As there is no neutral conductor being installed on the load side of the service disconnecting means, there is no grounded circuit conductor provided for the loads, and the only grounded conductor used is the equipment bonding conductor. If, however, a neutral conductor is intended to be used in circuits on the load side of the service disconnecting means, this neutral conductor is a grounded circuit conductor, as it carries the current, and it must be sized in accordance with rule 4-022.
Hopefully, figures 1 and 2 helped the reader understand most of the terminology in Rule 10-204, but what about system grounded conductors, and why these conductors are referred in plural? System grounded conductors described in Rule 10-204(4) are actually neutral conductors of the solidly grounded system established at the secondary of the supply transformer. As a Rule 10-204(4) deals with installation of grounded conductors in parallel runs, and such parallel runs must comply with Rule 12-108, this terminology is obviously used in a plural form.
Rule 10-204(2) could appear to be confusing, as it uses term grounded circuit conductors in a plural form. The fact is that this Rule deals with interconnection of neutrals (grounded circuit conductors) derived from two different systems. An example of such interconnection could be a transfer switch installed between a solidly grounded system of a normal power supply and a solidly grounded system derived from an emergency power supply (from a generator). It should be noted that if a ground-fault protection is provided at the solidly grounded system derived from the normal power supply (where a ground connection to the system grounding electrode is established), then use of the grounding electrode at the emergency power supply shall be avoided if a 3-pole transfer switch is utilized in the transfer scheme. This is extremely important; otherwise, a portion of a fault current at the load may travel to the grounding electrode at the generator, thus compromising operation of the ground-fault protection at the normal supply. In this case generator neutral must be isolated from the generator frame, and the generator frame must be bonded by the equipment bonding conductor.
Figure 3. Two different three-phase 4-wire solidly grounded systems with neutrals interconnected in a transfer switch
Figure 3 illustrates the essence of Rule 10-204(2).
Let’s analyze term common system grounding conductor used in Rule 10-206(1)(b).
This Rule governs grounding connections requirements when in addition to the main solidly grounded system that supplies the building, there could be other solidly grounded systems created by the step down transformers fed from the main distribution system.
Each new solidly grounded system established on a secondary of such step down transformer must be grounded in accordance with Rule 10-204(1)(c). However, Rule 10-206 recognizes that such grounding connection may be made not necessary by installing a separate grounding electrode at the secondary transformer (where new system is initiated), but by connecting a neutral point of the newly established system to the grounding conductor of the main solidly grounded system that supplies the building. Such grounding conductor would be a common system grounding conductor.
Figure 4 shows main solidly grounded system that supplies the building (system 1) and solidly grounded systems 2 and 3 created by the secondary of step down transformers in the building. The figure also illustrates that the neutral of system 2 is connected to the service grounding electrode by a common system grounding conductor. System 3 is provided with its own grounding electrode, and this electrode is interconnected with the electrode at the service by a bonding conductor conforming to Rule 10-702(b).
The code users should be aware that Rules 10-204 and 10-206 have been recently clarified by the S/C on Section 10, and that the S/C recommendation has been approved by the Technical Committee responsible for CE Code, Part I for adoption of these clarifications and for their inclusion into the 21st edition of the CE Code (CEC 2009).