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Fusible resistor power rating: 14 watts = too little

Sourcing fusible resistors above 7 watts seems impossible. Thus a parallel resistance network was used. Testing with 2 parallel pieces of type FW70A each 9.1 ohms rated at 7 watts was too little power for the transformer with up to 1218 A spikes. Where 4 parallel AC05 fusible resistors of 22 ohms rated 5 watts (P40), 20 watts in total, seems to be powerful enough.

1. To operate safely the schematic requires an overheating protection device for the resistor. In case the timer relay fails to actuate the contactor, all transformer standby consumption current flows via the resistor and the resistor will get very hot quickly (example standby consumption of the 80 kVA transformer is 0.24 kW where the resistor is only rated for .05 kW). For example use fusible wirewound resistors, like Bourns FW70A series, Panasonic ERQX, Vishay Draloric AC05-CS, Vitrohm CRF-500, etcetera.
2. Apply Nomex/Insultherm or equivalent fiberglass insulation sleeving to the 4 wires on the primary side between contactor and cable protection devices. Those cables would otherwise be unprotected.

### Fusible resistor power rating: 14 watts = too little

Sourcing fusible resistors above 7 watts seems impossible. Thus a parallel resistance network was used. Testing with 2 parallel pieces of type FW70A each 9.1 ohms rated at 7 watts was too little power for the transformer with up to 1218 A spikes. Where 4 parallel AC05 fusible resistors of 22 ohms rated 5 watts (P40), 20 watts in total, seems to be powerful enough. Fusible resistors cause higher peaks then non inductive wirewound.

1. To operate safely the schematic requires an overheating protection device for the resistor. In case the timer relay fails to actuate the contactor, all transformer standby consumption current flows via the resistor and the resistor will get very hot quickly (example standby consumption of the 80 kVA transformer is 0.24 kW where the resistor is only rated for .05 kW).
2. Don't use fusible wirewound resistors, like Bourns FW70A series, Panasonic ERQX, Vishay Draloric AC05-CS, Vitrohm CRF-500, etcetera. They cause high current on one phase, thus resulting in standby tranformer current flowing through it. The result is melted/burning wire connectors before the wirewounds have more then 100 kΩ resistance.
3. Better use a 4 pole MCB instead of the depicted 4 single pole MCB's to prevent safety issue #2.
4. Apply Nomex/Insultherm or equivalent fiberglass insulation sleeving to the 4 wires on the primary side between contactor and cable protection devices. Those cables would otherwise be unprotected.

Note2: Since the wirewound resistor was replaced with 4 parallel 15 ohms (AC05W15R00J) fusible wirewound resistors, thus 3.75 ohms, the inrush current is even lower then before. Highest inrush was (only) 7 amps and most of the times inrush is below 2 amps. (Path to ground will have a lower resistance then with the previous measurements, because ground water level is now around 1.5 meters below surface).

By trial and resistive inrush current measurements at 100 ksamples per second, I can measure less frequent high peaks at longer delays, though high inrush current peaks do still occur at 0.55s.

Because the MTR17-TAB-U240-208 timer relay is rated at 8 amps and magnetic circuit breakers in D-8A are expensive to source, a lower value D-6A breaker was used. Even with 0.55 seconds delay the L3 D-6A breaker tripped once where L1 and L2 inrush measurements were at 99 amps. Though better choose a B characteric. It turns out that D-6A doesn't protect the 8A relay in case of short circuit. Where a B-32A breaker was able to protect IKA-232 relays.

For improved uptime I'd rather accept 99 amp peaks over 45 amps, when the circuit can be protected with 32A breakers instead of 6A breakers. Therefore I chose to replace 100 ms delays of the MTR17-TAB-U240-208 timer relay with 15...25 milliseconds make delays of the 32A IKA232-20/230V relay.

In my case the primary side no load phase average measured around 2.2 Amps. Optimal neutral line resistance was calculated according to part II to be 8.97 Ohms:

No clue whether that resistor value should include or exclude the line to ground resistance of the earth rod. Although my measurements suggests the calculated resistance value includes the full circuit including star point earth rod resistance. With 8.5 Ohms inserted in the circuit the maximum inrush phase current was measured at 99 Amps and a D-6A breaker tripped (once during 60 test runs). Lowering the resistance to 4.5 Ohms reduced the peak values and didn't trip the D-6A breaker during 697 test runs. Lowering to 3.644 Ohms seems to slightly increase inrush L2-L3 current peak values.

Note: I didn't measure neutral to earth circuit resistance, though suspect 5 Ohms of resistance in the earthing rod at the utility company transformer according to that company its public documents for contracts, which states "...stop adding earth rods as soon as the resistance is lower then 5 Ohms". At the time of the measurements soil is dry to a lack of rain and ground water level is rather low at 3.3 meters below surface.

Sourcing fusible resistors above 7 watts seems impossible. Thus a parallel resistance network was used. Testing with 2 parallel pieces of type FW70A each 9.1 Ohms rated at 7 watts was too little power for the transformer with up to 1218 Amps spikes. Where 4 parallel AC05 fusible resistors of 22 Ohms rated 5 watts (P40), 20 watts in total, seems to be powerful enough.

This energy saving design powers down 4 (timer) relays N-L1-L2-L3 after enabling the contactor. This is done using a make-before-break H3DT-A2 timer relay and DILM150-XHI11 auxiliary its NC terminals on the contactor. Thus after switching, the only power consumed is by the contactor (2.4W) and the H3DT-A2 timer relay (0.6W). Start is issued by making an external relay. In this case the relay of a solar DC to AC inverter.

1. To operate safely the schematic requires an overheating protection device for the resistor. In case the timer relay fails to actuate the contactor, all transformer standby consumption current flows via the resistor and the resistor will get very hot quickly (example standby consumption of the 80 kVA transformer is 0.24kW where the resistor is only rated for .05kW). For example use fusible wirewound resistors, like Bourns FW70A series, Panasonic ERQX, Vishay Draloric AC05-CS, Vitrohm CRF-500, etcetera.
2. Apply Nomex/Insultherm or equivalent fiberglass insulation sleeving to the 4 wires on the primary side between contactor and cable protection devices. Those cables would otherwise be unprotected.

By trial and resistive inrush current measurements at 100 ksamples per second, I can measure less frequent high peaks at longer delays, though high inrush current peaks do still occur at 0.55 s.

Because the MTR17-TAB-U240-208 timer relay is rated at 8 A and magnetic circuit breakers in D-8A are expensive to source, a lower value D-6A breaker was used. Even with 0.55 seconds delay the L3 D-6A breaker tripped once where L1 and L2 inrush measurements were at 99 A. Though better choose a B characteric. It turns out that D-6A doesn't protect the 8 A relay in case of short circuit. Where a B-32 A breaker was able to protect IKA-232 relays.

For improved uptime I'd rather accept 99 A peaks over 45 A, when the circuit can be protected with 32 A breakers instead of 6 A breakers. Therefore I chose to replace 100 ms delays of the MTR17-TAB-U240-208 timer relay with 15...25 milliseconds make delays of the 32A IKA232-20/230V relay.

In my case the primary side no load phase average measured around 2.2 A. Optimal neutral line resistance was calculated according to part II to be 8.97 ohms:

No clue whether that resistor value should include or exclude the line to ground resistance of the earth rod. Although my measurements suggests the calculated resistance value includes the full circuit including star point earth rod resistance. With 8.5 ohms inserted in the circuit the maximum inrush phase current was measured at 99 A and a D-6A breaker tripped (once during 60 test runs). Lowering the resistance to 4.5 ohms reduced the peak values and didn't trip the D-6 A breaker during 697 test runs. Lowering to 3.644 ohms seems to slightly increase inrush L2-L3 current peak values.

Note: I didn't measure neutral to earth circuit resistance, though suspect 5 ohms of resistance in the earthing rod at the utility company transformer according to that company its public documents for contracts, which states "...stop adding earth rods as soon as the resistance is lower then 5 ohms". At the time of the measurements soil is dry to a lack of rain and ground water level is rather low at 3.3 meters below surface.

Sourcing fusible resistors above 7 watts seems impossible. Thus a parallel resistance network was used. Testing with 2 parallel pieces of type FW70A each 9.1 ohms rated at 7 watts was too little power for the transformer with up to 1218 A spikes. Where 4 parallel AC05 fusible resistors of 22 ohms rated 5 watts (P40), 20 watts in total, seems to be powerful enough.

This energy saving design powers down 4 (timer) relays N-L1-L2-L3 after enabling the contactor. This is done using a make-before-break H3DT-A2 timer relay and DILM150-XHI11 auxiliary its NC terminals on the contactor. Thus after switching, the only power consumed is by the contactor (2.4 W) and the H3DT-A2 timer relay (0.6 W). Start is issued by making an external relay. In this case the relay of a solar DC to AC inverter.

1. To operate safely the schematic requires an overheating protection device for the resistor. In case the timer relay fails to actuate the contactor, all transformer standby consumption current flows via the resistor and the resistor will get very hot quickly (example standby consumption of the 80 kVA transformer is 0.24 kW where the resistor is only rated for .05 kW). For example use fusible wirewound resistors, like Bourns FW70A series, Panasonic ERQX, Vishay Draloric AC05-CS, Vitrohm CRF-500, etcetera.
2. Apply Nomex/Insultherm or equivalent fiberglass insulation sleeving to the 4 wires on the primary side between contactor and cable protection devices. Those cables would otherwise be unprotected.