Grounding and Bonding of a Separately Derived System
A new commercial building has a 500 kVA, 600V/347V transformer supplying a 400A panelboard. The transformer is located outdoors, and the panelboard is indoors, 30 meters away. The electrical contractor installs a grounding electrode conductor (GEC) from the transformer neutral to a ground rod at the transformer location. At the panelboard, the neutral is bonded to the panel enclosure and a separate GEC is run to the building steel. The inspector notes that the bonding jumper at the transformer is missing and questions the grounding arrangement. The contractor argues that the neutral is grounded at the panelboard, so the transformer does not need a separate grounding. The building has a concrete-encased electrode and a metal water pipe. The soil resistivity is high, and the ground rod resistance is 150 ohms. The contractor did not perform a continuity test on the bonding jumper.
1. Is the contractor correct that the transformer neutral does not require a separate grounding electrode conductor? Explain the requirements for grounding a separately derived system.
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No, the contractor is incorrect. A separately derived system, such as a transformer, must be grounded in accordance with the Canadian Electrical Code. The neutral point of the transformer must be connected to a grounding electrode conductor that runs to a grounding electrode. The code requires that the grounding electrode conductor be sized per Table 16 and connected to the grounding electrode system. The transformer location has a ground rod, but its resistance is 150 ohms, which may not be adequate. However, the code does not specify a maximum resistance; it requires a connection to a suitable electrode. The contractor installed a GEC at the transformer, but omitted the bonding jumper between the neutral and the grounding electrode conductor at the transformer. This bonding jumper is required to establish the neutral-ground bond at the source. The panelboard also has a neutral-ground bond, which creates a parallel path for neutral current, a violation. The correct approach is to bond the neutral to ground only at the transformer (the source) and use a separate equipment grounding conductor to the panelboard, with the neutral isolated at the panelboard. The contractor must install the bonding jumper at the transformer and remove the neutral-ground bond at the panelboard. The grounding electrode system must include the concrete-encased electrode and metal water pipe, as available, and the ground rod can be supplementary.
2. What are the consequences of the missing bonding jumper and the multiple neutral-ground bonds, and what steps should the contractor take to correct the installation?
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The missing bonding jumper at the transformer means the neutral is not properly bonded to ground at the source, which can cause the neutral to float and create unsafe voltage conditions during faults. The multiple neutral-ground bonds (at transformer and panelboard) create parallel paths for neutral current, leading to objectionable current on grounding conductors and metal enclosures, increasing shock hazard and electromagnetic interference. To correct, the contractor must: 1) Install a properly sized bonding jumper at the transformer between the neutral and the grounding electrode conductor, sized per Table 16 based on the service conductor ampacity. 2) Remove the neutral-ground bond at the panelboard and ensure the neutral is isolated. 3) Verify that the equipment grounding conductor between the transformer and panelboard is continuous and sized per Table 16. 4) Test the grounding electrode system for continuity and ensure all electrodes are bonded together. 5) Confirm that the ground rod resistance of 150 ohms is acceptable; if not, additional electrodes may be needed. The inspector should re-inspect after corrections.