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Grounding and Bonding of Separately Derived AC Systems
By Mike Holt
When you hear "separately derived systems (SDS), do you automatically think of generators? If you don't work with generators, you may think SDS is not relevant to you. However, that thought may conflict with the reality of your situation.
Here's a frequently overlooked fact: All transformers, except autotransformers, are separately derived. This is because the primary circuit conductors have no direct electrical connection to the secondary circuit conductors.
Here's another frequently overlooked fact: Not all generators are separately derived. A generator supplying a transfer switch that opens the grounded neutral conductor or if there is no grounded neutral conductor is separately derived [250.20(D) FPN 1]. See Figure 250-71. But often generator setups don't switch the grounded neutral conductor in the transfer switch.
To determine if you have an SDS, start with the definition given in Article 100. An SDS is a premises wiring system whose power derives from a source other than a service. It also has no direct electrical connection (including a solidly grounded circuit conductor) to supply conductors originating from another system (Figure 250-70).
System Bonding Jumper
In Part 3 of this series, we learned you can't make neutral-to case (load side) bonds [250.24 (A)(5)]. We also learned this rule provides an exception for SDS, if you follow the requirements of 250.30(A)(1).
Size the system bonding jumper per Table 250.66 based on the area of the largest ungrounded secondary conductor [250.28(D)].
Question: What size system bonding jumper do you need for a 45 kVA transformer, where the secondary conductors are 3/0 AWG (See Figure 250-75)?
(a) 4 AWG (b) 3 AWG (c) 2 AWG (d) 1 AWG
Answer: (a) 4 AWG (Table 250.66).
Where you run an equipment bonding jumper to the secondary system disconnecting means, size it per Table 250.66 based on the area of the largest ungrounded secondary conductor.
Question: What size secondary equipment bonding jumper do you need for a nonmetallic raceway containing 350 kcmil secondary conductors (Figure 250-30A2 CC250-10.cdr)?
(a) 1 AWG (b) 1/0 AWG (c) 2/0 AWG (d) 3/0 AWG
Answer: (b) 1 AWG (Table 250.66).
Where you run SDS conductors in parallel, install a secondary equipment bonding jumper in each raceway (or cable) and size it per Table 250.6 based on the area of the largest ungrounded secondary conductor [250.102(C)].
Grounding Electrode Conductor (GEC)
For each SDS, ground the grounded neutral terminal (typically X0) to a suitable grounding (earthing) electrode of a type identified in 250.30(A)(7). Size the secondary system grounding (earthing) electrode conductor per 250.66 based on the total area of the largest ungrounded secondary conductor (Figure 250-81).
To prevent objectionable current flow in metal parts of electrical equipment, metal piping, and structural steel, terminate the GEC to the same point on the SDS where you installed the system bonding jumper (Figure 250-78).
Note the exceptions:
Connect grounding (earthing) electrode tap conductors to the GEC without splicing the common GEC. Install the GEC per 250.64-the basic requirements are that the GEC be:
If you run the GEC in metal enclosures, make them electrically continuous from the point of attachment to cabinets (or equipment) to the GEC [250.64(E)]. Bond building structural steel and all metal piping to an effective ground-fault current path, per 250.104(D).
If you have multiple SDS, you can ground the grounded neutral terminal (X0) of each SDS to a common GEC. But, the GEC and grounding (earthing) electrode tap must comply with 250.30(A)(4)(a) through (c) (Figure 250-30A4 CC250-11.cdr). We can summarize those as:
Grounding (Earthing) Electrode
Terminate the GEC to a grounding (earthing) electrode as close as possible-and preferably in the same area as-the system bonding jumper. The grounding (earthing) electrode must be the nearest:
If neither of these electrodes is available, use one of the following:
Observe the FPN in 250.30(7), to ensure quick removal of dangerous voltage from a ground fault. Bond metal water piping and structural steel in the area served by an SDS to the grounded neutral conductor at the SDS, per 250.104(D).
Grounded Neutral Conductor
If you install the system bonding jumper at the secondary system disconnecting means instead of at the source of the SDS, follow these requirements (Figure 250-84):
You cannot size the grounded neutral secondary conductor smaller than 1/0 AWG [310.4]. Remember, the grounded neutral conductor serves as the effective ground-fault current path.
Because SDS are pervasive, it's important to know-and correctly implement-SDS requirements. A mistake in the grounding and bonding of an SDS can prevent the clearing of a fault and it can allow dangerous potential to build on metal parts of the electrical system. Your new knowledge of SDS grounding and bonding requirements allows you to prevent both problems.
Mike Holt Comment: Summary from my best selling book, Understanding Grounding versus Bonding.
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