Marine switchboards

djd

Member
The question I have is why a donut would be around all three phase conductors and the neutral. This is done in a switchboard for a dock installation , all the switchboards are being replaced feeding the individual slips for the yachts. In the switchboard they have shunt trip breakers that are controlled by sensors. The purpose of donuts is unknown exactly they do have a small indicator board .
 

charlie b

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Staff member
Location
Seattle, WA
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Electrical Engineer
The question I have is why a donut would be around all three phase conductors and the neutral. This is done in a switchboard for a dock installation . . . .
If the total current sensed by this donut is not exactly zero at all times, it means that current is flowing back to the source via some other path (i.e., perhaps involving water). 555.3 requires a trip at 30 ma in such cases. My guess is that this is the reason. But it would need more than a small indicator board. It would need to include a trip of the main breaker on the switchboard.
 

djd

Member
my question was why are all three phases and nuetral in the donut , i have never seen that never seen that.
 

charlie b

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Staff member
Location
Seattle, WA
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Electrical Engineer
my question was why are all three phases and nuetral in the donut , i have never seen that never seen that.
I thought I had answered that. I will try again, with more detail.

Any current that leaves the source on Phase A should return to the source on either Phase B or Phase C or the neutral. Under normal circumstances, the mathematical sum of these four currents is exactly zero at every instant in time. That will cause the total magnetic field seen by the donut to be exactly zero at every instant in time. If the donut senses a net magnetic field that is not zero, it would mean that some current is returning to the source by a path other than the phase conductors or the neutral. That indicates a ground fault.
 

Russs57

Senior Member
DJD, by now you should understand what the purpose is and what it is being used for. I'll offer something you "might" need to be concerned with as I have seen mistakes made.

A lot of shunt trip coils can only handle being energized for a very brief period of time. You can wire in series with a normally open auxiliary contact if the breaker has one. The ground fault module may take care of this for you but I wouldn't assume so.

Personally, I'd pay and get a shunt trip coil that can handle being energized continuously (but only energize it until breaker trips). I see no downside (but there could be one I don't know about) and I would think it would last longer (but I may be wrong). Just make sure these get tested regularly as I have seen a good deal of failure on shunt trips. Certainly not my preferred way of doing ground fault protection. More like the right choice for an "emergency power off" button behind a glass door that gets used very rarely.
 

djd

Member
I thought I had answered that. I will try again, with more detail.

Any current that leaves the source on Phase A should return to the source on either Phase B or Phase C or the neutral. Under normal circumstances, the mathematical sum of these four currents is exactly zero at every instant in time. That will cause the total magnetic field seen by the donut to be exactly zero at every instant in time. If the donut senses a net magnetic field that is not zero, it would mean that some current is returning to the source by a path other than the phase conductors or the neutral. That indicates a ground fault.
Thank
 
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djd

Member
I thank for your information , I always tell these young guys in the trade theory is important as I was like they. I have just never seen this application and thank you again.
 
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