Ahead/before and behind/after

[ggunn]

In discussions I am having with an AHJ rep, he refers to a disconnect which is between a PV meter and the POI as being ahead of the meter despite the fact that it is on the load side of the meter; is this the usual way of looking at it? Considering the direction of current flow, it is "behind" the meter under normal conditions and "ahead" of the meter if there is a fault on the inverter side.

 

[zbang]

IMNSHO, we need to ditch the words before and after for this use because they're so ambiguous. Maybe just talk about "between X and Y" or something like that.

 

[ggunn]

IMNSHO, we need to ditch the words before and after for this use because they're so ambiguous. Maybe just talk about "between X and Y" or something like that.

I agree, but the AHJ guy I am talking to uses them.

 

[Hv&Lv]

In discussions I am having with an AHJ rep, he refers to a disconnect which is between a PV meter and the POI as being ahead of the meter despite the fact that it is on the load side of the meter; is this the usual way of looking at it? Considering the direction of current flow, it is "behind" the meter under normal conditions and "ahead" of the meter if there is a fault on the inverter side.

If its on the load side of the meter, we call it after.

 

[ggunn]

If its on the load side of the meter, we call it after.

Of the utility meter, yes, but what about a PV meter? The load sides of a utility meter and a PV meter point toward each other.

 

[wwhitney]

There's variation in usage, and hence confusion. IMO the standard should be with respect to the primary power source. So for grid-tied PV, it should aways be relative to the grid. Ahead of X would mean between X and the grid; behind X would the other side of X.

In other words, if I understand the OP correctly, I like the AHJ's terminology. You could think of it as conventional power flow, even through during periods of (high) PV generation, the actual power flow is in the other direction. Just like conventional current flow is in terms of positive charge carriers, even though electrons are usually the charge carriers.

But as there is no agreement on a standard, you basically have to specify your terminology whenever you use it.

Cheers, Wayne

 

[zbang]

It's kind of like "normally open" as a door interlock switch when it's supposed to be closed in "normal" operation (door closed).

 

[ggunn]

There's variation in usage, and hence confusion. IMO the standard should be with respect to the primary power source. So for grid-tied PV, it should aways be relative to the grid. Ahead of X would mean between X and the grid; behind X would the other side of X.

In other words, if I understand the OP correctly, I like the AHJ's terminology. You could think of it as conventional power flow, even through during periods of (high) PV generation, the actual power flow is in the other direction.

Actually you do not understand me correctly; in the conductors I am speaking of if the PV is producing anything at all the power flow is toward the service. The power is never away from the service except at night when the inverter and/or monitoring hardware may draw a minuscule amount of power, or if there is a fault.

 

[wwhitney]

Actually you do not understand me correctly; in the conductors I am speaking of if the PV is producing anything at all the power flow is toward the service. The power is never away from the service except at night when the inverter and/or monitoring hardware may draw a minuscule amount of power, or if there is a fault.

I got that part.

But it makes no sense to me to use "before" and "after" with respect to local current flow. IMO they should be global terms, e.g. with respect to the grid connection. So before would mean towards the grid connection in all case.

As there's no standard, you can just skip before and after and use obviously global terms like "grid side"

Cheers, Wayne

 

[solarken]

Actually you do not understand me correctly; in the conductors I am speaking of if the PV is producing anything at all the power flow is toward the service. The power is never away from the service except at night when the inverter and/or monitoring hardware may draw a minuscule amount of power, or if there is a fault.

What is the context of the discussion with the AHJ? Are you trying to decide what is the load side and line side of the switch, in relation to the Meter/PV and the Grid interconnection point? I guess I am confused by ahead of or behind the meter discussion, as there seem tpo be two meters in your scenario. If the connectivity is in the order: Inverter - PV Meter - Disconnect - Grid connection - Utility Meter, and if there is no Main disconnect between the grid connection point and the Utility Meter, then the Solar disconnect is on the line side (or supply side) of the Main Service Disconnect, but on the load side (behind) of the utility meter. It is also on the Grid side of the PV meter. This disconnect should have it's line side terminals connected toward the grid connection point and the load side terminals toward the PV meter. The other thing that should be considered is that Inverters are no longer always sources. The SolarEdge Energy Hub includes capability of directly connecting their EV charger, so then there can be a huge load with power flow going to the inverter from the grid connection point. Or for hybrid inverters they may charge a DC coupled battery at night, again changing the inverter from a source to a load.