Circuit Breaker Series Rating "through" transformer?

[dinos]

Does anyone know if series rated combinations apply when the line/load devices are on opposite sides of a transformer?

For example:

at transformer 480V primary/; GE type SFH
at transformer 208V secondary/; GE type TQD

Per the GE's guide "DET008 R12 1211", the TQD is series rated for 22,000A at 240V but I believe the SFH would have to be on the "same side" of the transformer (also applied at 240V).

If there is a specific documentation that discusses this issue that would be helpful (my search results have netted nothing thus far).

 

[kwired]

Does anyone know if series rated combinations apply when the line/load devices are on opposite sides of a transformer?

For example:

at transformer 480V primary/; GE type SFH
at transformer 208V secondary/; GE type TQD

Per the GE's guide "DET008 R12 1211", the TQD is series rated for 22,000A at 240V but I believe the SFH would have to be on the "same side" of the transformer (also applied at 240V).

If there is a specific documentation that discusses this issue that would be helpful (my search results have netted nothing thus far).

Assuming no transformer impedance, and 240 volts secondary, a 480 volt primary will only see half the current as what is on the secondary. 208 secondary will have a little higher current for same VA.

 

[Jraef]

You are correct in that a series rating would not apply here. But it may not be that the TQD is series rated in that situation, it may be just that the Prospective Short Circuit Current starts over on the secondary side of the transformer and is mostly based on the transformer size and impedance (starting from the PSCC of the primary) and what OCPD is on the primary has little bearing on that.

So is the PSCC on the secondary more that 10kA? If not, it doesn’t matter. If so, then the breaker on the primary is not going to help.

 

[oldsparky52]

I don't know if the series rating goes through a transformer, but would a 480 primary develop more than 22kaic?

 

[kwired]

You are correct in that a series rating would not apply here. But it may not be that the TQD is series rated in that situation, it may be just that the Prospective Short Circuit Current starts over on the secondary side of the transformer and is mostly based on the transformer size and impedance (starting from the PSCC of the primary) and what OCPD is on the primary has little bearing on that.

So is the PSCC on the secondary more that 10kA? If not, it doesn’t matter. If so, then the breaker on the primary is not going to help.

Right, unless the transformer is high capacity in comparison to the supply source, you likely have smaller VA rating and high enough impedance that the available short circuit current on secondary is somewhat low to begin with. Service supplied by 2mVA 480 transformer but a 15 kVA 208 transformer to serve general lighting/receptacles in a specific area is going to have a low SSC in comparison to the service anyway.

 

[dinos]

I don't know if the series rating goes through a transformer, but would a 480 primary develop more than 22kaic?

Transformer impedance is 3.2% and the calculated fault current at the 208/120V secondary is exceeding 10k. An "infinite" primary source calculates results in worst case 625A/3.2%=19,530A, but within the actual distribution the fault currents are down around 12,000A.

 

[dinos]

So is the PSCC on the secondary more that 10kA? If not, it doesn’t matter. If so, then the breaker on the primary is not going to help.

Transformer impedance is 3.2% and the calculated fault current at the 208/120V secondary is exceeding 10k. An "infinite" primary source calculates results in worst case 625A/3.2%=19,530A, but within the actual distribution the fault currents are down around 12,000A.

 

[kwired]

Transformer impedance is 3.2% and the calculated fault current at the 208/120V secondary is exceeding 10k. An "infinite" primary source calculates results in worst case 625A/3.2%=19,530A, but within the actual distribution the fault currents are down around 12,000A.

if impedance in your distribution gets you below the rating of your device, series ratings don't matter anymore, your max available current is below the general rating of the device.