I know that the neutrals and grounds must be isolated in subpanels. What is the potential consequence if they are not.
So how do eddy currents affect the residential electrical service? Is it a reduction of potential capacity or something more dangerous?
You need to Google this and spend some time trying to understand it.
someone could be killed
If they’re connected on both ends downstream of a service disconnect then the neutral current will have two paths to travel (they’re connected in parallel) on the neutral and on the EGC. Current should never flow on the EGC unless there is a fault condition. Sorry Roy but this has little to do with eddy currents.
Thanks everyone… I will do more research but you guys gave me what I need for now!
As I understand, if there were surge of current due to say, a lightning strike, the potential for current to flow into the ground and then in turn to the neutrals, having been bonded, it could potentially destroy your wiring throughout the house… Not to mention all of your electrical appliances.
Thanks Robert been 55 +years since I was at school
This is a very long and complex topic so bare with me. It took me a while to grasp it even partly.
Basically a neutral to ground bond has 2 major issues among others:
One is should the neutral fail or open, in a 3 wire feed this would cause everything connected to the subpanels ground/neutral buss to become live. Everything such as metal frames of appliances will be energized. An oven from the main panel and say a refrigerator from the subpanel will have 120 volts potential between them. Basically anything from the main panel will be zero volts as well as piping, HVAC ect while anything from the subpanel will be 120 volts to ground. This is the exact reason why 3 wire feeds to ranges and dryers are no longer allowed in new installations. A live ground is both a fire and a shock/electrocution hazard.
In a 4 wire feed should the neutral fail the ground wire will carry it all, and often it is undersized since the NEC knows its function is to clear faults only rather then continuously carry current. A 100amp subpanel is ok with a #8 copper ground while the neutral is often #3 or #2. Big difference in size.
Even if the neutral does not fail another set of issues crop up.
The grounding system will now have current dividing between it and the neutral. This is because current takes all paths, and because the neutral is not a perfect zero ohms and the grounding system is another path, current will take detours.
That detour current further has 2 results:
It will travel unrestrained on duct work, pipes, conduit ect. When ever current travels it produces a 60Hz magnetic field. So does wire in conduit or cable, but normally one hot wire is carrying for example 10amps one way while the other neutral wire carries 10amp the other way. Because this current is equal yet opposite in magnitude it cancels out producing almost no detectable magnetic field. GFCIs work this way to sense an imbalance. However, that is not the case in objectionable current. It will produce intense magnetic fields along its entire run. The occupants will be exposed to elevated magnetic fields, but health effects aside there is still the issue of magnetic induction. Conduit is ferromagnetic, and because it is carrying stray current it will begin to heat from magnetic induction. That heating is a fire risk since it will not allow conductors inside the conduit to dissipate heat, on the contrary it adds heat. The magnetic fields can also cause sensitive electronics to have issues.
This is the reason why when conductors are paralleled the NEC requires that one of each phase and neutral be in the same conduit if it is ferromagnetic.
Another example is older codes: they allowed for a 3 wire feed to a subpanel provided in a detached structure provided no metallic paths existed between the home and structure.
Magnetic induction aside, current is current. A loose connection on conduit fittings can spark and arc from that current. In addition to that any current produces a voltage gradient. A voltage gradient cause the grounding system to loose zero reference. In a nut shell voltage potentials will exist between the subpanel and main panel. Even if no other code violations exist, that difference can still shock a person under the right conditions. It will also cause issues with electronics like audio equipment, ie the classic “dirty ground” since voltage is present on the ground when it should not be. This graphic may help:
To put it simply electricity is meant to flow only on insulated conductors and the grounding system is only for emergencies like a hot wire touching an appliance frame.
Ok, hopefully its not to confusing.
The same thing would happen to a service panel.
Robert M has the best explanation.
The service panel should be grounded separately via a ground rod or mechanical bond to the rebar. That should take care of any surges and prevent current from back feeding via ground and neutral(s) being bonded.
The ground loop is also an interference issue within the house circuitry and is controlled via a separation of ground and neutrals.