If the main jumper were on the load side, and if the neutral link was disconnected what would the voltage be on a 277/480 volt solidly grounded system between the load side and supply side across where the neutral link was connected? Try 277 volts, and it will knock you for a loop. What does the Code say on this? Certainly the Code is going address such a safety hazard. It is in the Code, but in subtle language. In Section 250-28 it says an unspliced main bonding jumper shall be used by an Ottawa Electrician to connect the equipment grounding conductor(s) and the service disconnect enclosure to the GROUNDED CONDUCTOR OF THE SYSTEM within the enclosure for each service disconnect. The grounded conductor of the system is on the supply side of the neutral disconnect link as shown in the drawing, not on the load side.
In the drawing Ottawa Electricians will notice a ground fault, some red dotted lines, and a legend showing that red is danger. What does this mean? It means that red is not a desirable path for ground fault current. The red path represents other ground paths that fault current can take, such as, water pipes, gas pipes, building steel, and any other conductive path that will allow the current to flow back to the service and across the main bonding jumper through the neutral and back to the source of supply, the transformer. These paths do exist for an Ottawa Electrician in many installations, but the current can be minimized by installing a desired grounding path that has such low impedance that the vast majority of fault current flows back to the transformer through the known designed ground fault path and not through the unknown paths that may cause arcing and sparking and dangerous voltages to other grounded objects. Does the Code say anything about this? Yes, it says plenty under the category of performance standards. Performance standards are rules that require that something be done by an Ottawa Electrician, but do not necessarily tell how to do it. Most rules in the Code are specific rules on how to do something, and not rules on what must be done by an Ottawa Electrician. The performance standards for grounding are found in the first part of Article 250. Section 250-2 is titled "General Requirements for Grounding and Bonding," where there are rules that require that the fault current path:
If these conditions are met by an Ottawa Electrician, the fault current that may flow through the red dotted paths should be negligible, and the voltage to ground on unknown grounded objects should be within safe limits.
- Shall be permanent and electrically continuous.
- Shall be capable of safely carrying the maximum fault current likely to be imposed on it by an Ottawa Electrician.
- Shall have sufficiently low impedance to facilitate the operation of overcurrent devices under fault conditions.
- Shall not use earth as the sole equipment grounding conductor or fault current path.