Shared neutrals giving weird results with the Ideal SureTrace!?

[dword32]

I'm getting some interesting results when tracing circuits with a shared neutral.

The panel is 208V 3 phase. The breakers are wired so that #1, #3, #5 correspond to Phase A, B, C, respectively, and it continues on down....

There is a single neutral wire used for each set of 3 breakers. For example, I see a white neutral marked as "#1 #3 #5" and "#7 #9 #11" and so on...

When I take the Ideal SureTrace transmitter and connect plug it into a receptacle which ties back to breaker #3, and then run the SureTrace receiver up and down the panel, breakers #1 or #5 actually register as a higher signal than the correct breaker #3. I can only assume this is due to the shared neutral?

To further clarify:
If I connect the SureTrace transmitter in the "direct ground" scenario where the (+) jack connects to receptacle's Line, and the (-) jack connects to the outlet's neutral, then breaker #1 reads as the highest value, even though the correct one is #3.

If I connect the SureTrace transmitter in the "separate ground" scenario where the (+) jack connects to the receptacle's Line, and the (-) jack connects to the ground pin of a receptacle on a different breaker, then breaker #5 reads as the highest value, even though the correct one is #3.


I'm having a hard time envisioning why breakers #1 or #5 would read higher than the correct #3 breaker. I welcome any input! Thanks!

 

[roger]

A MWBC is a single circuit so the Sure Trace is actually correct even though the leg/phase being traced may be lower than the others. The current in a 4 wire wye circuit is shared with #1, #3, and #5 (A,B,C) so it only makes sense that a circuit tracer will see all.

Roger

 

[dword32]

A MWBC is a single circuit so the Sure Trace is actually correct even though the leg/phase being traced may be lower than the others. The current in a 4 wire wye circuit is shared with #1, #3, and #5 (A,B,C) so it only makes sense that a circuit tracer will see all.

Roger

Would you say I'm being too critical of the SureTrace in this situation? In this MWBC configuration, would most professionals consider it a job well done that it was at least able to narrow it down to that cluster of 3 breakers? Or should it still be able to narrow it down to the specific breaker which leads to the receptacle where the transmitter is plugged in?

 

[dpcarls1598]

Funny you mention that...
I have found the Sure Trace to be very accurate on 3 phase loads when little to no load is on them. At our buildings (schools), I try to work when the building is minimally or completely unoccupied and the accuracy is very good.
If I’m working with staff or students in the building, the accuracy tends to go down and while I haven’t ever metered it, I find the circuit with the larger load on it tends to indicate higher even though it’s not the circuit I’m on.

 

[hillbilly1]

The Amprobe CT 326 does not have that problem, but then it cost a lot more too! You can even find mistapped neutrals going to the wrong panels, or neutrals from two different panels tied together. The drawback for this accuracy, is it doesn’t trace dead circuits.

 

[synchro]

Was the #3 breaker turned ON when you had the transmitter connected to the receptacle on the #3 circuit? If the #3 breaker was OFF and #1 and #5 were ON then that could definitely explain why #3 had a weaker signal.

Did you try putting the receiver probe next to the wire connected to the breaker, or just the breaker itself? Also the orientation of the probe can make a difference so it may help to try rotating it.

 

[dword32]

Was the #3 breaker turned ON when you had the transmitter connected to the receptacle on the #3 circuit? If the #3 breaker was OFF and #1 and #5 were ON then that could definitely explain why #3 had a weaker signal.

Did you try putting the receiver probe next to the wire connected to the breaker, or just the breaker itself? Also the orientation of the probe can make a difference so it may help to try rotating it.

All 3 breakers were ON while the transmitter was connected to the receptacle on the #3 circuit. I would have expected #3 to therefore give me the highest reading, but it was either #1 or #5 (depending on whether it was direct ground or separate ground connection).

 

[zbang]

If there are loads on the MWBC, the signal could potentially leak through the loads to other legs depending on all the impedances involved. It could also leak by induction between legs.

I'm with Roger- if you get any of the breakers of the MWBC, you've got it.

(All three breakers would have to be on, or off, at once for a properly-done MWBC. And if it's one neutral for three hot legs, it pretty much has to be one.)

 

[LarryFine]

Some breaker finders require you to sweep the entire panel once, then again without letting off the trigger to locate the signal.

 

[dword32]

If there are loads on the MWBC, the signal could potentially leak through the loads to other legs depending on all the impedances involved. It could also leak by induction between legs.

I'm with Roger- if you get any of the breakers of the MWBC, you've got it.

(All three breakers would have to be on, or off, at once for a properly-done MWBC. And if it's one neutral for three hot legs, it pretty much has to be one.)

You do have a point about all three breakers needing to be ON or OFF at the same time. Isn't it current code that a 3 pole breaker should be used so that all 3 circuits are tied together?

 

[zbang]

Isn't it current code that a 3 pole breaker should be used so that all 3 circuits are tied together?

A 3-pole or 3 single-pole with handle ties, I'd have to look at the specifics, and I believe it has been for some time now.

That said, who wrote this??? (from 2008, 225.33)

(B) Single-Pole Units. Two or three single-pole switches or breakers capable of individual operation shall be permitted on multiwire circuits, one pole for each ungrounded conductor, as one multipole disconnect, provided they are equipped with identified handle ties or a master handle to disconnect all ungrounded conductors with no more than six operations of the hand.

(where did that come from?)

OTOH, in 240.15

(1) Multiwire Branch Circuit. Except where limited by 210.4(B), individual single-pole circuit breakers, with or without identified handle ties, shall be permitted as the protection for each ungrounded conductor of multiwire branch circuits that serve only single-phase line-to-neutral loads.

Looks like there are several other places that agree with 225- handle ties required on single-pole breakers.