Grounding(bonding) for 480/240 Single Phase Transformer.

[electricalRookie]

Hi all,

I am reading the NEC code 2020

250.2B:
(B) Alternating-Current Systems 50V to 1,000V.
The following systems must be grounded (connected to a grounding electrode) where the neutral conductor is used as a circuit conductor:
(1) Single-phase systems
(2) Three-phase, wye-connected systems.
(3)Three-phase, high-leg delta-connected systems.

here it says it must be grounded however in 250.21, it seems like I can still technically not ground my system...

(B) Ground Detectors. Ungrounded alternating-current systems from 50V to 1,000V or less are not required to be grounded if:
(1) The ungrounded systems operating between 120V and 1,000V as permitted in 250.21 (A) have ground detectors installed on the system.
(2) The ground detection sensing equipment is connected as close as practicable to where the system receives its supply.

here are my questions:
1. Can I unground my single-phase system but ground all the metal cases?
2. what is the pros and cons if I unground this system?
3. if I measure the L1/L2 to the ground, what voltage I would expect?

 

[jim dungar]

1. You cannot ungrounded your system if loads are connected to the neutral, 250.2(B)(1).
2. There is no advantage to ungrounding it. One disadvantage is the ability to feed line to neutral loads.
3. With an undefined reference the measured voltage will depend on the impedance of your measurement device and the coupling capacitance of the system.

 

[electricalRookie]

1. You cannot ungrounded your system if loads are connected to the neutral, 250.2(B)(1).
2. There is no advantage to ungrounding it. One disadvantage is the ability to feed line to neutral loads.
3. With an undefined reference the measured voltage will depend on the impedance of your measurement device and the coupling capacitance of the system.

Hi Jim dungar, thanks for your reply!

per your answer, that means for a single-phase system, we must ground it without any exception per NEC 250.2(B)(1).

just imagine this is an ungrounded system but all the metal cases are connected to the primary side ground. if one line is accidentally touches the metal case, can we consider it now grounded? what are the dangers with one hot touching the metal case which is connected to the primary side ground?

250.21 says I can unground it if I add a ground fault detector, I wonder if this only applies to 3 phase systems but not 1 phase system.

Thanks!!!!!

 

[jim dungar]

Yes an unintentional ground is still a ground.
The primary and secondary grounds are always connected together.
You can ungrounded any system that does not employ a neutral and exceeds 150V to ground.

 

[electricalRookie]

Yes an unintentional ground is still a ground.
The primary and secondary grounds are always connected together.
You can ungrounded any system that does not employ a neutral and exceeds 150V to ground.

Thanks again!
You mentioned neutral in the previous post, in this case, Can I assume all single-phase systems have a neutral meaning they must be grounded according to NEC 250.2

 

[jim dungar]

Thanks again!
You mentioned neutral in the previous post, in this case, Can I assume all single-phase systems have a neutral meaning they must be grounded according to NEC 250.2

Not all single phase systems are 3 wire, although most are.
Not all single phase systems are less than 150V to ground.

 

[Little Bill]

If the grounded/neutral conductor from the POCO is brought to the service entrance, whether you use it or not, the system is grounded as the POCO grounds it at the pole or transformer.