Lets assume you have a step down transformer feeding a subpanel. No continuous neutral between primary side and secondary side of transformer.
You must bond the neutral on the secondary side of your transformer to the equipment case. I understand that this is to keep all of your voltages relative to the same ground voltage throughout the building.
But, what if you didn't bond the neutral? So what if your air conditioning units operating at 480V and your computers operating at 120V were at really strange voltages relative to each other?
The only danger I see if if somebody was holding on to two live neutrals on different systems at the same time. But that's probably not a good idea anyway. I'm sure there's a down side but I can't see what it is. Two of us at work have been discussing this and I knew somebody where would know the answer.
If you didn't bond the nuetral, there would be no way for a ground fault to clear an OCPD. The fault path must return to the source, and you just eliminated that path, by not bonding the nuetral to the grounds at the source. The source being the Seperately Derived System, ie the transformer.
I'm sure someone will correct me...
#129860 - 10/06/0511:31 AMRe: Neutral bonding in separately derived systems
If you don't bond the neutral, then you are quite correct: a ground fault _won't_ cause the OCPD to trip.
On the other hand, if the secondary of the transformer is not ground referenced, then a _single_ ground fault won't have much if any current flow; this is what is known as an 'ungrounded system', and some systems are intentionally designed this way. Dig though the NEC and section on grounding and you will see that some systems are simply _not_ required to be grounded. The idea is that the system will continue to function with a single ground fault. Generally the way that this works is that you have circuits set up to detect ground faults, and in the event of a ground fault you work to isolate the faulted circuit, then actually shut the system down at a convenient time.
There are many problems with this approach. In particular, you always have _some_ coupling to ground, even if it is only capacitive coupling. In the event of an intermittent ground fault, you can actually develop phase to ground voltages that are several times the normal phase to phase voltage; in your example, it doesn't really matter if your 480V HVAC circuits are 277V, 277V,277V or 0V,480V,480V, but it very much does matter to the insulation system if you ever manage to hit 1600V,2000V,2000V
The 'modern' approach to this sort of system is the 'high impedance grounded' system, where the neutral is bonded to ground via a resistor. The resistor is sized to permit a significant but not particularly large current to flow in the event of a ground fault; enough to solidly tie the voltages to a ground reference, and enough to detect the ground fault, but not enough to cause significant damage. The system that I work on this past spring used a 50 ohm resistance to ground on a 480V/277 system, giving about 5A of ground fault current.
#129861 - 10/06/0502:15 PMRe: Neutral bonding in separately derived systems
Jon, I think he was asking about bonding the nuetral in a wye system.
Your post reflects ungrounded delta systems in a industrial enviroment with appropriate supervision and a series of fault lights, so if a fault occurs on one leg, the system doesn't crash so to speak. maintenance personell have to clear the first fault before the second happens.
In a wye system, you must have a grounded conductor.
Is this correct?
#129863 - 10/06/0509:51 PMRe: Neutral bonding in separately derived systems
If you are operating under the NEC, the only systems _permitted_ to be built as ungrounded systems are delta systems in a particular voltage range. It would violate code to have install an ungrounded wye system.
However an ungrounded wye system would _function_ as described.
I presume that ungrounded wye systems are not permitted because of safety issues. Many phase to neutral loads are designed with the expectation that the neutral will be grounded, eg. the screw shells of lamp holders should be at ground potential.
Impedance grounded systems are often based on wye secondaries, but are not permitted to feed line-neutral loads.
#129864 - 10/06/0511:08 PMRe: Neutral bonding in separately derived systems
I ran into a situation similar to this while working in a high rise office building down town.We were remodeling do to flood damage caused by the sprinkler system.Part of one floor was finished and sublet to a legal office.I get a call saying sparks are flying and eqiupment is damaged.I had checked the outlets days before and every thing tested properly so i told the tenant it was probably faulty eqiupment.I got out my tester to check the voltage in a 120v receptical and my jaw about hits the floor .I have 200v hot to neutral and 120v neutral to ground.Back at the panel i get the same readings.The panel is fed from a step down transformer 480 delta primary/208 wye secondary.This was a pre existing installation with no bonding jumper.We shorted the ground to neutral and the voltage dropped from 120v to 3v and did not return.So we scheduled a shutdown and installed a bonding jumper in the transformer.This solved the problem but i wondered why it never showed up before.Then it dawned on me .I had removed two 240v outlets in the copier room that were wired incorrectly.They were fed with threewire bx and the neutral wire was connected to the grounding screw effectively becoming the bonding jumper for the service.Sometimes two wrongs can make a right .Well allmost but it was a great learning experience because i to wondered about the importence of bonding separately derived systems.Ps i think the transient voltage was static from the new carpet install.
#129866 - 03/28/0601:30 PMRe: Neutral bonding in separately derived systems