Swapping neutral and red phase wires in 3-phase 400/230V system

[mtkhan]

Hi All. Theoretically, If there was an accidental swapping of red phase and neutral cables at the point of connection of a 3-phase 4-wire 400/230V system, what would happen downstream to 3-phase & single phase loads once power is switched back on? 

If we assume the system is fed from a standard delta-star 11kV/400V 1MVA municipal transformer.  This was an actual municipal technician mistake! and it got me thinking what would've been the outcome

Thank you

 

 

[Jaco De Jongh]

2 hours ago, mtkhan said:

If there was an accidental swapping of red phase and neutral cables at the point of connection of a 3-phase 4-wire 400/230V system,

"Theoretical" Whatever single phase loads were connected to the remaining two phases would have been fed with 380V instead of 220V, and three phase loads should have tripped on some  unbalance condition. 

Practically it would have been almost impossible to switch on the supply if any downstream neutrals were bonded to earth.(all depending of the meaning of your "point of connection"). It would have meant live connected to earth directly and some protection would have tripped somewhere.  

[Gerrie]

I think it would be possible to switch on the municipal breaker feeding, because in theory you only swop the two colors of red and black downstream the neutrals are now live and red (live) are now neutral.

This will cause single phasing on 3 phase motor loads and probably trip the motor protection breakers due to overload.

Your red phase equipment would still have the 230V across terminals. And red phase equipment should still work but with wrong polarity.

Single phase loads on white and blue phase will have 400V across terminals and  some equipment might burn out.

 

[Jaco De Jongh]

1 hour ago, Gerrie said:

I think it would be possible to switch on the municipal breaker feeding, because in theory you only swop the two colors of red and black downstream the neutrals are now live and red (live) are now neutral

You are welcome to come and try it at the Mini Sub feeding my block, My neutral is bonded to earth at my DB board. So is the neutral at both my neighbors houses. I would love to see you switch on the breaker. 

[mtkhan]

thanks guys.  yeah I'm also struggling to see how you'd avoid a direct phase to ground fault immediately once you switch on main breaker since the red phase will then be connected directly to earth (via neutral bar bonded directly to earth in what  I assume is a TNC-S earthing arrangement) and shouldn't this immediately should trip main breaker on overcurrent no?

 

I've attached photos taken of municipal main breaker supply when it was still swapped incorrectly, and tried to sketch out the scenario as well where I do see there'll be 380-400V on the remaining phases with the red phase simply oppsoite polarity. But like you pointed out, I struggle to see the main breaker not trip immediately if this was the case.

 

Thank you

 

 

[Jaco De Jongh]

5 minutes ago, mtkhan said:

I've attached photos taken of municipal main breaker supply when it was still swapped incorrectly,

We dont know what black and red was connected to where they originate. It might be that they have connected a live to Black and neutral to red, Or the cable might have been incorrectly joined in the past. Colors isnt always an indication of what the wire is, although it should be.  

[Gerrie]

5 hours ago, mtkhan said:

thanks guys.  yeah I'm also struggling to see how you'd avoid a direct phase to ground fault immediately once you switch on main breaker since the red phase will then be connected directly to earth (via neutral bar bonded directly to earth in what  I assume is a TNC-S earthing arrangement) and shouldn't this immediately should trip main breaker on overcurrent no?

 

I've attached photos taken of municipal main breaker supply when it was still swapped incorrectly, and tried to sketch out the scenario as well where I do see there'll be 380-400V on the remaining phases with the red phase simply oppsoite polarity. But like you pointed out, I struggle to see the main breaker not trip immediately if this was the case.

 

Thank you

 

 

This looks like there might of been a open circuit in the cable on the red conductor, so just to get the customer going they temporary swapped black and red till it could be fixed.

But if they forgot to swop the red on the other end once cable is fixed i’m sure they will need some sunglasses when switching on again. unless there is no earth loop impedance back to the transformer.