Isolated Ground Termination Point

[rpruitt]

I am of the belief that an isolated ground bus should be connected to the system with an insulated ground wire to the point where the earth ground wire from the ground rod connects to the service entrance ground/neutral. I was told it is just fine to just make this connection inside the building at the neutral bus of the main panel. The IG panel is a sub-panel two feet from the main panel making the IG bus connection only 30” long then connected to the main panel neutral lug bonding all together. I do not see where this accomplishes anything at all. It is the same as running the branch circuits (IG’s) to the main panel instead and making their connections at that point. The current flowing flowing in the grounded conductor (neutral) between the panelboard and separately derived system (transformer) would cause instability (noise) in the ground reference. I am having a little trouble understanding exactly what the NEC is directing on this. I read it as the connection should be at the service entrance but I am being overridden. This system is 208v 3 phase and the building has more than one occupancy (separate metering). Can someone clarify this for me?

 

[dpc]

I'm not familiar with the concept of an isolated ground panelboard. Is this even allowed by the NEC?

The NEC definitely does allow isolated ground receptacles and requires that the insulated grounding conductor be terminated at an equipment grounding terminal of the service or derived system BUT, this conductor cannot pass through the building to the outside without being grounded. This is typically done at the service entrance panelboard. Running the isolated grounding conductor outside the building directly to the ground rod, is probably a violation. Keep in mind that typical service may have multiple grounding electrode conductors in addition to the outside ground rod.

 

[peebee]

Per NEC 150.146(D), Isolated Receptacles:

". . . This grounding conductor shall be permitted to pass through one or more panelboards without connection to the panelboard grounding terminal as permitted in 408.20, Exception, so as to terminate within the same building or structure directly at an equipment grounding conductor terminal of the applicable derived system or service."

So:
1. Termination at the neutral bus is not permitted. You must terminate at the "ground terminal" (ground bus).
2. You are right that having a separate IG panel makes little difference over just running all your IG grounds to the main panel. It can be a convenient way to segregate circuits, though.
3. The current flowing in the IG conductors should be negligable. That's the whole point of having an IG system, it's essentially a single-point-grounding or star-connected-grounding type systems which eliminates ground loops. Without loops, there can be no current. So there's no real issue of a voltage developing between the SE ground bus and the IG ground bus as there's no current flow.
4. Note that the connection must be made to "an equipment grounding conductor terminal of the applicable derived system or service". That may or may not be your SE panel.
5. It's not clear to me if your "main" panel and "SE" panel are the same panel or not. Please clarify. If they are on the same separately derived system, though, it would be acceptable to terminate your IG conductor at the ground bus of either panel.

Hope this helps.

 

[HydroInspector]

I know this thread deals primarily with the US, but here is how we deal with the situation in Canada.

Rule 10-906 Bonding Conductor Connection to Circuits and Equipment

(6) A bonding jumper shall be installed to connect the bonding conductor to the grounding terminal of a receptacle and in such a manner that disconnection or removal of the receptacle will not interfere with, or interrupt, grounding continuity.

(7) In the case of metallically enclosed systems where the grounding path is provided by the metal enclosure, a bonding jumper shall be installed to bond the grounding terminal of the receptacle to the enclosure.

(8) Notwithstanding Subrules (6) and (7), the bonding jumper, in the case of receptacles having grounding terminals isolated from the mounting strap required for special equipment, shall be permitted to be extended directly back to the distribution panel.

(9) Notwithstanding Rule 10-808, electronic equipment rated to operate at a supply voltage not exceeding 150 volts-to-ground and which requires a separate bonding conductor shall be permitted to be bonded to ground by an insulated conductor extending directly back to the distribution panel, provided that:

(a) The separate bonding conductor is enclosed in the same raceway or cable containing the circuit conductors throughout the length of that cable or raceway; and

(b) The separate bonding conductor is sized not less than that given in Table 16 for each leg of the run, determined by the size of the overcurrent protection for the circuit conductors; and

(c) The bonding requirements of Rules 10-302 and 10-400 are met.

Rationale for Rule 10-906.

When we lose a circuit conductor connection, we know immediately because the light goes off or the electrical equipment stops, or perhaps a fault occurs and a fuse or circuit breaker operates. When we lose a bonding conductor, we do not know until there is a fault, and at that time the loss of the bond may result in serious damage, shock, or fire. We have therefore taken every precaution to install the bonding conductor in such a way that it will not be accidentally or inadvertently disconnected. We make the connection of the bonding conductor by secure methods that do not depend on solder, and we fasten it with a screw or device that is used for no other purpose. We make sure that the bond is present and properly connected at every point at which it could be required and interconnected in such a manner that it will not be disconnected if a receptacle or other device is removed. We take care that the bond will remain continuous even if it is connected to or run through removable covers, and we do not depend on the mounting screw of a receptacle as a bonding connection.

In Subrule (8) we have permitted a separation between the bonding of the outlet box and the ground connection to a receptacle intended for use with electrical equipment that may be sensitive to power disturbances such as noise and voltage spikes carried over the bonding system. Voltage spikes, which are usually caused by the switching of large loads, can damage such electronic equipment as computers or corrupt the computer software, causing system shutdown and loss of data. This is also the rationale of Subrule (9), which applies not just to the bonding conductors to receptacles that supply data processing and similar equipment but to the bonding conductors for any circuit operating at not more than 150 V that supplies such equipment. In both these cases we have permitted a separate and dedicated bonding conductor to be run from the sensitive electrical equipment, directly back to the distribution panelboard.

Intent for Rule 10-906.

When we connect a bonding conductor, we wish to provide as secure a connection as possible. We do this by

(a) using only positive connection methods;

(b) making a connection that does not depend on solder (we can accept solder if the connection is secure and then soldered; under the Part II Standards, a wire that passes through a hole and is bent is considered to be secure);

(c) using a dedicated ground screw in each outlet box (ie, a screw that is used for no other purpose);

(d) bringing the bonding conductor into every outlet box, whether it is metal or nonmetallic, and securing it so that it is available for the bonding of any device installed there;

(e) installing the bond in such a way that the removal of a cover will not break the bonding circuit, even when the cover has the circuit conductors running through it (this will usually require a jumper running with the circuit conductors or between the cover and the enclosure);

(f) connecting the bonding conductor in any box in such a way that the removal of a device will not interrupt the bonding circuit;

(g) providing a bonding conductor from the outlet box to a receptacle if the bonding of the system depends on metal conduit; or

(h) permitting two separate bonding systems to exist, one for the raceway and one for the receptacle or other outlet connection, where, because of the possibility of damage to data processing equipment or to computer programs, a dedicated insulated bonding conductor is used.

Intent for Rule 10-906(8).

For receptacles, removal of the bonding jumper is permitted in accordance with Subrule 10-906(8), where the voltage-to-ground does not exceed 150 volts-to- ground, the receptacle is supplying sensitive electronic equipment, and the conditions of Subrule (9)(a), (b) and (c) are met.

Intent for Rule 10-906(9)

This Subrule is intended to permit insulated bonding conductors, serving a receptacle or hard wired equipment, to pass through panelboards, extending back to the main distribution switchboard and connected to the case or grounding bus at that point in the main distribution switchboard. The Subrule does not permit the separate bonding conductor to pass through the main distribution switchboard to a separate grounding electrode.

In this case, Subrule (9) applies to insulated bonding conductors that are used as the sole bonding means for exposed metal parts on sensitive electrical (electronic) equipment, and only in systems operating at voltages-to-ground of 150 V or less.

Tony Moscioni
Electrical Inspector
Electrical Safety Authority