Pvfarmer questions

[PVfarmer]

Grounding electrodes are primarily for surge and lightning protection and are of no use for faults. Grounded systems are required to have an adequate low impedance path for the fault current to return to the source.

I'm missing one part here.
Say a hot wire in an outlet melts, so there is now current flowing through the EGC from that outlet to the main ground bar, which is bonded with the neutral bar in the MSP.
The fault current then goes to the neutral of "the source", meaning the XO of the xfmr, and back to the breaker to trip it.

So...how does the fault become part of the "overall flow", yet somehow get back to the breaker? Does the neutral/fault current get "added to" the line current in the xfmr in order to cause the trip?

the fault current flows through the appliance ground wire to the service panel where it joins the neutral path, flowing through the main neutral back to the center-tap of the service transformer. It then becomes part of the overall flow, driven by the service transformer as the electrical "pump", which will produce a high enough fault current to trip the breaker.

 

[augie47]

The melting outlet would not in itself cause any overcurrent device to operate (might be a selling point for AFCI ). If a phase to ground fault resulted then, yes, the increase in current flowing back thru the equipment ground, thru the neutral bond back to the transformer would cause the breaker to trip.

 

[PVfarmer]

If a phase to ground fault resulted

thru the neutral bond back to the transformer would cause the breaker to trip.

Right, I meant "with a phase-ground fault resulting".

And ok! Thanks. So would you say the "extra" fault current going *to* the xfmr..."cancels" the "extra" going to the faulted breaker, or does it..."become" that extra, in the xfmr windings themselves?

Or...I suppose neither is a possibility?

 

[PVfarmer]

Power Company transformers do have primary over-current protection.... They certainly can not be depended upon for secondary protection.

Pardon, it was too late to edit- but about that secondary protection and "In the event of a ground-fault on the line-side of the service "...

as the OP is wondering- is the kAIC of the main panel breaker involved in that? As in a service rated breaker must have a 10kAIC rating (or 14kAIC or more depending on service amperage), and depends on the POCO MV OCPD among (many) other things?

Also, if there is a short on the "line side", line side meaning between the LV POCO xfmr connections and the main customer OCPD, the customer OCPD is supposed to handle that by tripping/blowing, not the POCO OCPD?

And... if the MV OCPD is one of these variables you mentioned, it does affect the kAIC of main customer OCPD?

What I'm trying to get at is-
if you have PV connected "line-side", as in connected in some fashion to a point between the meter and main panel/main breaker...
the PV and the main OCPD should have kAIC ratings equal to or (probably) greater than these numbers below (for example)- you could have a fused PV switch and main breaker with same amp rating (say 125A), but different kAIC ratings?

"but there are a lot of variables which will determine those devices offering any protection for secondary faults."
(Local) POCO example below.
---
For equipment rating purposes, the following tables list the maximum fault currents available at the Company’s transformer secondary terminals.

Three Phase Pad Mounted Transformer Available Fault Current
amperes Symmetrical Fault Current:
75kVA
208/120 =20,900
480/277 =10,000

150kVA
208/120 =34,700
480/277 =15,100

 

[augie47]

Lot for my ancient brain to try and filter at one time... few thoughts below:

Pardon, it was too late to edit- but about that secondary protection and "In the event of a ground-fault on the line-side of the service "...

as the OP is wondering- is the kAIC of the main panel breaker involved in that? As in a service rated breaker must have a 10kAIC rating (or 14kAIC or more depending on service amperage), and depends on the POCO MV OCPD among (many) other things?
The available current should certainly be taken into account in selecting the overcurrent device but that would not effect the fault path or current flow.

Also, if there is a short on the "line side", line side meaning between the LV POCO xfmr connections and the main customer OCPD, the customer OCPD is supposed to handle that by tripping/blowing, not the POCO OCPD?
Disagree.. iF there is a phase to phase or phase to neutral short on the LINE side of the curomer's OCPD, that device will not detect the short.
And... if the MV OCPD is one of these variables you mentioned, it does affect the kAIC of main customer OCPD?
The MC OCPD could affect the available fault current and thus the AIC rating of the customers OCPD device. I rarely see POCO take that into consideration in providing SCA numbers.
What I'm trying to get at is-
if you have PV connected "line-side", as in connected in some fashion to a point between the meter and main panel/main breaker...
the PV and the main OCPD should have kAIC ratings equal to or (probably) greater than these numbers below (for example)-Agree.. you could have a fused PV switch and main breaker with same amp rating (say 125A), but different kAIC ratings?

"but there are a lot of variables which will determine those devices offering any protection for secondary faults."
(Local) POCO example below.
---
For equipment rating purposes, the following tables list the maximum fault currents available at the Company’s transformer secondary terminals.

Three Phase Pad Mounted Transformer Available Fault Current
amperes Symmetrical Fault Current:
75kVA
208/120 =20,900
480/277 =10,000

150kVA
208/120 =34,700
480/277 =15,100

 

[PVfarmer]

Lot for my ancient brain to try and filter at one time... few thoughts below:

1 The available current should certainly be taken into account in selecting the overcurrent device but that would not effect the fault path or current flow.

2 Disagree.. iF there is a phase to phase or phase to neutral short on the LINE side of the curomer's OCPD, that device will not detect the short.

3 The MC OCPD could affect the available fault current and thus the AIC rating of the customers OCPD device. I rarely see POCO take that into consideration in providing SCA numbers.

4 the main OCPD should have kAIC ratings equal to or (probably) greater than these numbers below (for example)-Agree..

Hey, thanks for the answers and splitting the thread up like that.

The line side fault path not changing in #1 is the reason the customer device in #2 doesn't detect the fault, and #2 is also why there has to be a POCO OCPD.

Thanks for mentioning SCA, I found this article, and glad you agree on #4.
If there was a serious problem on the line side, and no MV side OCPD, something might very well explode? I think I've got it.
http://ecmweb.com/content/short-circuit-current