Fused Disconnect Switches and Transformer Secondary Protection

[julianov]

Hello All


I've reviewed this single line diagram, which shows two transformers being used because the motor is far away and there isn't enough room to use a large wire size due to the existing conduit constraints. However, I have some concerns: the panel only has a circuit breaker (CB) and the main disconnect switch at the motor is not fused.

Does the NEC require that the disconnect switch at the motor be fused? Additionally, is it necessary for the secondary side of the transformers to have a disconnect switch or a circuit breaker?

I know it's not ideal because if there is a fault, the breaker in panel A will trip, making it difficult to identify where the fault is. But regarding the code, what does it say?

 

[jim dungar]

Selective coordination is generally a design decision and not mandated by NEC except in 'emergency' systems.

But your secondary conductors need to have protection per 240.21(C). If you are trying to skip this protection your step up transformer needs to be a 3W delta configuration and you will need to provide ground fault detection.

 

[julianov]

Thanks.

The transformers have a "high voltage side" of 600 V configured in Delta (Δ) and a 208 V side configured in Wye (Y). Therefore, the connections between the transformers are in Delta. This seems compliant with the code. However, the conductors between the transformers extend over 500 feet, and the secondary conductors to the motor disconnect switch extend over 15 feet. Note that the secondary of the second transformer, connected to the main disconnect switch, is in Y configuration and lacks overcurrent protection. This absence of overcurrent protection on the secondary side of the transformer to the motor is what concerns me.

The transformer configuration for the entire system is Y-Δ and Δ-Y.

 

[augie47]

Your concern is warranted... the wye secondary should have overcurrent protection,
Hopefully you can select a single OCP which is suitable to protect the transformer and the motor.

 

[don_resqcapt19]

Thanks.

The transformers have a "high voltage side" of 600 V configured in Delta (Δ) and a 208 V side configured in Wye (Y). Therefore, the connections between the transformers are in Delta. This seems compliant with the code. However, the conductors between the transformers extend over 500 feet, and the secondary conductors to the motor disconnect switch extend over 15 feet. Note that the secondary of the second transformer, connected to the main disconnect switch, is in Y configuration and lacks overcurrent protection. This absence of overcurrent protection on the secondary side of the transformer to the motor is what concerns me.

The transformer configuration for the entire system is Y-Δ and Δ-Y.

The permission for the primary OCPD to protect the secondary conductors only applies to 3 wire delta to delta transformers.

240.21(C)(1) ...
Single-phase (other than 2-wire) and multiphase (other than delta-delta, 3-wire) transformer secondary conductors are not considered to be protected by the primary overcurrent protective device.

 

[julianov]

But your secondary conductors need to have protection per 240.21(C). If you are trying to skip this protection your step up transformer needs to be a 3W delta configuration and you will need to provide ground fault detection.

The permission for the primary OCPD to protect the secondary conductors only applies to 3 wire delta to delta transformers.

But why?

I have conducted several simulations with this Y-Δ transfomer to Δ-Y to resistive load:



If a short circuit occurs to ground in the secondary transformer, it may be possible to detect the failure in the primary of the first transformer.



The same applies in the case of a biphasic failure



I can also detect the failure if there is a short circuit between the wires.



No matter how many transformers I place in between, it seems that the fault will always manifest in the main primary. But in this case, we are discussing direct sequence. Could it be that protection is only allowed on the primary in Δ-Δ transformers because there might be something related to the third harmonic and homopolar currents?

 

[julianov]

I've just realized that with a Y-Δ transformer, the homopolar impedance is substantial, which would prevent any circulating homopolar current in the system. Therefore, I'm unsure why the NEC specifies that we can omit using OCPD in the secondary only for Δ-Δ transformers.

 

[jim dungar]

I've just realized that with a Y-Δ transformer, the homopolar impedance is substantial, which would prevent any circulating homopolar current in the system. Therefore, I'm unsure why the NEC specifies that we can omit using OCPD in the secondary only for Δ-Δ transformers.

There are very little engineering reasons behind many NEC rulings, instead the NEC is based on real world installations over the past 100 years. Wye-delta transformer arrangements ate not common in installations under the NEC.