Neutral and ground bonding

Status
Not open for further replies.
M

mathan1987

Member
Location
Seattle
Occupation
Electrical design engineer
Where do w normally bond the Neutral and ground in the Electrical distribution system. Why do we normally do it only at the service equipment not at the panels down stream. do we do Neutral and ground bonding at each step down transformer?
 
mathan1987 said:
Where do w normally bond the Neutral and ground in the Electrical distribution system. Why do we normally do it only at the service equipment not at the panels down stream. do we do Neutral and ground bonding at each step down transformer?
Click to expand...
You only bond neutral at one point. At the service disconnecting means.

Everything down stream from that is connected via a EGC in the event ungrounded conductor fails or comes into contact to normally non-current carrying metal parts.

The EGC is basically an extension from the neutral (effective ground fault current path) for the purpose of clearing faults and not leaving non current carrying metal parts energized.

If you are to connect the neutral conductor anywhere downstream from the service disconnect, the EGC will be carrying neutral current in parallel. Which would be called objectionable current and in violation of the NEC.

Step down transformers as in a SDS (separately derived system) you can either have the neutral to ground bond in the tranny, or have the MBJ in the first panel that is supply’s. (Speaking as in a standard delta-eye step down transformer.)
 
Dsg319 said:
You only bond neutral at one point. At the service disconnecting means.

Everything down stream from that is connected via a EGC in the event ungrounded conductor fails or comes into contact to normally non-current carrying metal parts.

The EGC is basically an extension from the neutral (effective ground fault current path) for the purpose of clearing faults and not leaving non current carrying metal parts energized.

If you are to connect the neutral conductor anywhere downstream from the service disconnect, the EGC will be carrying neutral current in parallel. Which would be called objectionable current and in violation of the NEC.

Step down transformers as in a SDS (separately derived system) you can either have the neutral to ground bond in the tranny, or have the MBJ in the first panel that is supply’s. (Speaking as in a standard delta-eye step down transformer.)
Click to expand...
what will happen if we were to connect neutral and ground in the downstream panels?
 
mathan1987 said:
what will happen if we were to connect neutral and ground in the downstream panels?
Click to expand...
The ground (EGC) and all other metal connected between the two panels will be in parallel to the neutral and have neutral current on it.
Mike Holt has some good images on objectionable current, look them up they’re good.
 
mathan1987 said:
what will happen if we were to connect neutral and ground in the downstream panels?
Click to expand...
Added: I'm addressing a 3-wire feeder here, not paralleled conductors.

Under normal conditions, nothing. Think about old three-wire major appliances where the neutral was permitted to perform grounding functions (not the other way around). A sub-panel in a detached structure is another example, allowed only if there was no other effective ground/zero-volts reference available.

Allowing one conductor to act as both neutral and EGC risks energizing exposed conductive surfaces in case that conductor fails, as with open neutrals. Sub-panels, grounding terminals, and even plumbing could become energized relative to nearby conductive surfaces that are still at or near zero volts.

The purpose of an EGC is so an accidental contact between a hot conductor and a conductive surface mimics a hot-to-neutral fault, so the OCPD trips before someone is hurt. The EGC system begins at the last point where the neutral is allowed to be uninsulated. That point "defines" zero volts for the premises.
 
LarryFine said:
Added: I'm addressing a 3-wire feeder here, not paralleled conductors.

Under normal conditions, nothing. Think about old three-wire major appliances where the neutral was permitted to perform grounding functions (not the other way around). A sub-panel in a detached structure is another example, allowed only if there was no other effective ground/zero-volts reference available.

Allowing one conductor to act as both neutral and EGC risks energizing exposed conductive surfaces in case that conductor fails, as with open neutrals. Sub-panels, grounding terminals, and even plumbing could become energized relative to nearby conductive surfaces that are still at or near zero volts.

The purpose of an EGC is so an accidental contact between a hot conductor and a conductive surface mimics a hot-to-neutral fault, so the OCPD trips before someone is hurt. The EGC system begins at the last point where the neutral is allowed to be uninsulated. That point "defines" zero volts for the premises.
Click to expand...

I would add that the EGC ensures that conductive surfaces are at zero volts even in normal operation, not just during a hot to neutral fault.
Using the neutral for bonding would result in some small voltage gradient of those surfaces simply due to voltage drop on the neutral. The level may or may not result in a hazard depending on many factors.
 
retirede said:
I would add that the EGC ensures that conductive surfaces are at zero volts even in normal operation, not just during a hot to neutral fault.
Using the neutral for bonding would result in some small voltage gradient of those surfaces simply due to voltage drop on the neutral. The level may or may not result in a hazard depending on many factors.
Click to expand...
Well, there is also the voltage on the service neutral and the utility primary neutral that is put on to the EGC via the main bonding jumper so there still a small voltage on the EGC under normal conditions. This potential would be from anything connected to the electrical grounding system and the earth...for example between a concrete basement floor and the EGC.
 
don_resqcapt19 said:
Well, there is also the voltage on the service neutral and the utility primary neutral that is put on to the EGC via the main bonding jumper so there still a small voltage on the EGC under normal conditions. This potential would be from anything connected to the electrical grounding system and the earth...for example between a concrete basement floor and the EGC.
Click to expand...

Very true.

I was mainly thinking of the case where the metallic plumbing system is bonded properly, the EGC ensures that exposed metallic parts of utilization equipment do not carry voltage with respect to the plumbing.

Our MGN utility system basically guarantees that there will be potential between the EGC and various points of “earth”.
 
"Step down transformers as in a SDS (separately derived system) you can either have the neutral to ground bond in the tranny, or have the *(MBJ) in the first panel that is supply’s. (Speaking as in a standard delta-eye step down transformer.)"

Replace *MBJ with SBJ since this is down stream from the MBJ
Key word here is system.
Very important difference.
Most knew what he meant since you trying to understand, clearly is important.
The take away would be do only in one place with an SDS.

At SDS, WYE secondary with neutral if there is a *Jumper from XO to the ground bar In the trans. Now you have Bonded the neutral point of the WYE secondary in the transformer (SDS) which a SBJ is used.

With this SDS code allows this SBJ to take place in one of two locations as mentioned above by others. Either the SDS (trans in this case) or the first means of disconnect down stream from this SDS.
Side bar an SDS does not always mean a transformer. Please set that aside however retain for later.

Since we're on the load side( down stream) from the MBJ it shall be refered to as an SBJ which allowes this option.
At first means this jumper can be used as long and no jumper is installed at the SDS. Now your SBJ is in the panel or switch. One location not both.
Now once a decision is made on location you move forward to the rest. Second would now be the GEC. This electrode conductor must go to the same location of the SBJ.
Layman terms, if you use the green screw (panel) no jumper or GEC at SDS SSBJ on secondary side and an EGC on primary and case/ core ground. No XO bond! One wire only to this terminal which goes to the first mean terminal, aka netutal bar in this case.


Doing this you can now understand the reason for the SSBJ.
It need to be sized for full load fault current of the system due to the ungrounded neutral conductor. After all this wire is not grounded yet if SBJ is at first means. We can not refer to it as a grounded (neutral)conductor untill we do so via the SBJ and an GEC.

You see now how the neutral in bonded only at one location and be maintained though out that entire system. Now we have a grounded netutal and a grounded system.

Great question by the way.
Thanks for asking
 
Status
Not open for further replies.
Top