An example would be single phase transformer. On the transformer's secondary you have no particular wire designated as a neutral. This means you need to pick one and ground it. This essentially assigns it as being the neutral.
You could also have a generator that once a transfer switch operates, cuts the utility and leaves you with a separately derived source. This source needs to still have proper grounding.
Any system that has no electrical connection to an other system is a separately derived system. Transformers are a magnetic connection, not an electrical connection; and most transformer connections create a separately derived system. The grounded-wye/grounded-wye connection does not create a separately derived system as there is a connection between the neutrals through ground (and an internal connection between H0 and X0 on may wye/wye transformers).
Generators switched through 4 pole transfer switches (or 3 pole and no neutral brought out of the generator) are separately derived while generators that are paralleled to the utility or switched with transfer switches that do not switch the neutral are not separately derived.
I have been under the impression that separately derived systems could be joined at the neutral to simplify switching (and in some instances protection) to meet the "One connection between ground and neutral" rule.
I may be mistaken and I am away from my books and can't research this just now.
Bill
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"Why not the best?"
Jimmy Carter
One rule to be aware of, and which likely varies by country, regards separation or interconnection of HV and LV earth electrodes if the separately derived system is an LV system fed from an HV network, as is the case in many distribution substations.
Generation will count as a separately derived system, certainly when it is operating in isolation, and the neutral and earthing arrangements need careful design especially if the set is also capable of parallel operation.
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A good example of this would be a PDU (power distribution unit)as typically seen in a data center. It takes 480Vac input, uses a transformer to reduce this to 120Vac and then provides a sets of single phase distribution panels. The 120Vac is considered a separately derived source and the PDU must have a bonded neutral and earth (chassis / safety) ground that provides a safe fault return path for devices supplied by the distribution panels.
Check out 2 gensets with a 3-pole xfer switch. Genset 1 gets bonded (N-G).. GENSET 2 gets no MBJ therenot seprately derived systems unless we install a 4-pole Xfer switch.. we would then bond both of them (N-G).. once the neutrals are switched they are separately derived and each need to be bonded accordingly.
check outthe attachment...
Re. Waross' statement "I have been under the impression that separately derived systems could be joined at the neutral to simplify switching (and in some instances protection) to meet the "One connection between ground and neutral" rule."
You're almost right -- except when you put the solid bond in there, you no longer have two separately derived systems.
For example, you could use a 3-pole transfer switch with solid neutral to switch between a 4-wire utility and 4-wire generator *so long* as the only N-G bond is at the utility. The utility is then the only separately derived system. The generator is *not* separately derived in this case, it shares a neutral with the utility.
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Separately derived systems are covered in Article 250, Grounding and Bonding, especially paragraphs 250.20 through 250.35.
If you can get your hands on a copy of the NEC Handbook rather than the plain-Jane NEC, it does a bit better job of describing separately derived systems.
The IEEE Green Book also does a nice job of covering separately derived systems.
