NEC 495.70 and 495.71 Clarification as to why a 3-phase solidly grounded wye source is required.

[mayanees]

Does anyone have familiarity with the requirement for electric boilers above 1000 Volts to be powered with a solidly ground wye system?
495.71 Electrical Supply System.
Boilers shall be supplied only from a 3-phase, 4-wire solidly grounded wye system, or from isolating transformers arranged to provide such a system. Control circuit voltages shall not exceed 150 volts, shall be supplied from a grounded system, and shall have the controls in the ungrounded conductor.

Solidly grounded is defined as not being connected through a resistor or impedance.

I could understand that if the controls faulted each phase for heating, that there may need to be a minimum ground fault current available, but I could also see how the controls could function line-to-line to vary the heating process.

Thanks for any insight.

 

[Sparky558]

Does anyone have familiarity with the requirement for electric boilers above 1000 Volts to be powered with a solidly ground wye system?
495.71 Electrical Supply System.
Boilers shall be supplied only from a 3-phase, 4-wire solidly grounded wye system, or from isolating transformers arranged to provide such a system. Control circuit voltages shall not exceed 150 volts, shall be supplied from a grounded system, and shall have the controls in the ungrounded conductor.

Solidly grounded is defined as not being connected through a resistor or impedance.

I could understand that if the controls faulted each phase for heating, that there may need to be a minimum ground fault current available, but I could also see how the controls could function line-to-line to vary the heating process.

Thanks for any insight.

What’s the question? Sorry just looking for clarity

 

[tom baker]

Are you asking for the theory of why solidly grounded?

 

[mayanees]

Are you asking for the theory of why solidly grounded?

Yes I suppose that's what I'm asking. Or perhaps why is this a requirement? Why solidly grounded?

 

[synchro]

Apparently electrode type steam boilers are inherently wye type loads, with the neutral connected to major components of the boiler including its outside shell. So the requirement for them to be solidly grounded is likely for safety reasons, and maybe for other issues as well.

From pg. 6 of the following document:
"Since both the nozzle stock and the counter electrode are in contact with the boiler shell, they form the common connection points of a ''Y” connected load."

https://watmfg.com/watmfg23082016/w...lectrode-Operation-and-Maintenance-Manual.pdf

 

[mayanees]

Apparently electrode type steam boilers are inherently wye type loads, with the neutral connected to major components of the boiler including its outside shell. So the requirement for them to be solidly grounded is likely for safety reasons, and maybe for other issues as well.

From pg. 6 of the following document:
"Since both the nozzle stock and the counter electrode are in contact with the boiler shell, they form the common connection points of a ''Y” connected load."

https://watmfg.com/watmfg23082016/w...lectrode-Operation-and-Maintenance-Manual.pdf

I've heard that this is common in the States, but that other countries don't need the solidly grounded centertap. I wonder what drove the NEC to publish that requirement.

 

[NVelect]

I've heard that this is common in the States, but that other countries don't need the solidly grounded centertap. I wonder what drove the NEC to publish that requirement.

Did you ever find an answer on why a solidly grounded 4W supply is a requirement of the NEC?

 

[mayanees]

Did you ever find an answer on why a solidly grounded 4W supply is a requirement of the NEC?

No, but one can assume that the NEC prefers grounded systems.

 

[joymon]

It really doesn’t make sense why the NEC would mandate the solidly grounded. With a high impedance ground the ground faults would be lower thus more safe I would think.

 

[mayanees]

It really doesn’t make sense why the NEC would mandate the solidly grounded. With a high impedance ground the ground faults would be lower thus more safe I would think.

...not to mention that arcing faults would be essentially eliminated, since most arcing faults start as phase-to-ground and that's tolerated in an impedance-grounded system. I worked for P&G and every large system (substation transformers 1500-2500 kVA) they installed was impedance grounded, and there was cost associated with that methodology. But they're a zero-risk company, and they concluded that arcing faults can be minimized with impedance grounding.

 

[joymon]

...not to mention that arcing faults would be essentially eliminated, since most arcing faults start as phase-to-ground and that's tolerated in an impedance-grounded system. I worked for P&G and every large system (substation transformers 1500-2500 kVA) they installed was impedance grounded, and there was cost associated with that methodology. But they're a zero-risk company, and they concluded that arcing faults can be minimized with impedance ground

At P&G did you ever encounter equipment like this connected directly to the impedance grounded system? E.g 13.8kV or 5kV motor drives?
I have really trying to understand why the NEC would mandate the 4W connection for the electrode boilers higher than 1000V yet for the ones below, you could get away with a 3W connection as long as the boiler is sufficiently insulated.

 

[mayanees]

At P&G did you ever encounter equipment like this connected directly to the impedance grounded system? E.g 13.8kV or 5kV motor drives?
I have really trying to understand why the NEC would mandate the 4W connection for the electrode boilers higher than 1000V yet for the ones below, you could get away with a 3W connection as long as the boiler is sufficiently insulated.

No, they were always 480/277V systems.

 

[texie]

While I have been fortunate to be involved with many facets of NEC things and even worked with pretty large boilers, I have never encountered an electric boiler of any great size. How common are these?

 

[joymon]

The Electrode type boilers, though available in low voltage ranges in Europe and the rest of the world, are primary of voltages higher than 1000V in North America. There have not been a lot of installs the last many years primarily because natural gas prices have been low. However with decarbonization and renewable integration, these units are starting to make a resurgence. From my research these units at voltages higher than 1000V, during normal operations have significant amounts of “leakage” currents and likely the reason why the NEC mandates the neutral connection for improved safety, as in the 490.70, but why wouldn’t impedance grounded options be a alternative is what escapes me.