Why we ground low voltage services

[mbrooke]

I just came across this from a utility's transformer standards PDF which I found rather eye opening. On page 67 it makes mention of a electrostatic charge with the secondary winding floating, which in theory makes sense considering there is a capacitive coupling between the primary and secondary windings in a typical transformer.

https://www.google.com/url?sa=t&rct...3czYWZ7aZPTLSIAEPwWoSg&bvm=bv.139782543,d.cGc

With that in mind, if secondary systems were left ungrounded, wouldn't the entire 120/240 volt secondary service be at 750 to 7000 volts potential above earth? But by grounding the secondary neutral point once at the pole and again at the main service disconnect, that induced voltage can the be "bled off", or more correctly use the earth as a return path back to POCO substation?


If that is true, then why or how does the NEC allow for an ungrounded service? :?

 

[zbang]

With that in mind, if secondary systems were left ungrounded, wouldn't the entire 120/240 volt secondary service be at 750 to 7000 volts potential above earth?

Well, yes, but there's also capacitance between the secondary-side conductors and actually grounded things that can pull that float voltage back down.

 

[JFletcher]

wag, but electrostatic charge would only be a danger to lineman. Avg person can build up 20kV dragging shoes across a carpet.

 

[Smart $]

wag, but electrostatic charge would only be a danger to lineman.

Any non-lineman working on the secondary also.

Avg person can build up 20kV dragging shoes across a carpet.

The danger posed is a matter of how much energy is stored.

 

[Smart $]

... If that is true, then why or how does the NEC allow for an ungrounded service? :?

The mentioned electrostatic charge is a result of transformer design and connection. Most designs do not leave an ungrounded conductor connected beyond the disconnecting means. Disconnect all ungrounded conductors simultaneously sound familiar?

 

[mbrooke]

The mentioned electrostatic charge is a result of transformer design and connection. Most designs do not leave an ungrounded conductor connected beyond the disconnecting means. Disconnect all ungrounded conductors simultaneously sound familiar?

It does, but a 7,000 volt charge might cause some insulation damage down the road.

 

[Smart $]

It does, but a 7,000 volt charge might cause some insulation damage down the road.

The 7kV electrostatic charge only exists when the CSP breaker contacts are open, the primary is energized, and the xfmr is wired as depicted in Figure 2... and if the service is complete, the service disconnecting means is open (i.e. no grounded-conductor loads are connected).

Wiring as depicted in Figure 3 reduces the charge by almost 90%. However, 750V is still in excess of 600V-rated wire... but how often is the transformer going to have the primary energized and the secondary disconnected as depicted?

 

[mbrooke]

The 7kV electrostatic charge only exists when the CSP breaker contacts are open, the primary is energized, and the xfmr is wired as depicted in Figure 2... and if the service is complete, the service disconnecting means is open (i.e. no grounded-conductor loads are connected).

Wiring as depicted in Figure 3 reduces the charge by almost 90%. However, 750V is still in excess of 600V-rated wire... but how often is the transformer going to have the primary energized and the secondary disconnected as depicted?

I guess then the question is: why does 750 volt go away with the CSP breaker closed?

 

[Smart $]

I guess then the question is: why does 750 volt go away with the CSP breaker closed?

The floating portion (Figure 3 winding CD) of the secondary is then connected to the grounded portion (Figure 3 winding AB).

 

[mbrooke]

The floating portion (Figure 3 winding CD) of the secondary is then connected to the grounded portion (Figure 3 winding AB).

Correct, to the gorunded portion. But my question is, what if those same pots fed a 240 volt ungrounded delta service?

 

[Smart $]

Correct, to the gorunded portion. But my question is, what if those same pots fed a 240 volt ungrounded delta service?

That's a valid question. Your bringing this up about the 120V connection is the first I've heard of it. I find it hard to speculate with any certainty that it would or would not occur for any ungrounded system.

Appears there is good reason to use grounding jumpers when performing maintenance tasks even when power is disconnected from the system. :happyyes:

 

[mbrooke]

That's a valid question. Your bringing this up about the 120V connection is the first I've heard of it. I find it hard to speculate with any certainty that it would or would not occur for any ungrounded system.

Appears there is good reason to use grounding jumpers when performing maintenance tasks even when power is disconnected from the system. :happyyes:

I agree in theory. But let me ask this: should the NEC forbid ungrounded services or require ungrounded services to be fed via a shielded transformer?

 

[Smart $]

I agree in theory. But let me ask this: should the NEC forbid ungrounded services or require ungrounded services to be fed via a shielded transformer?

While an electrostatic charge in the kilovolt range is certainly nothing to take lightly, I'd have to believe an ungrounded system posed an imminently greater danger to life and limb before banning or requiring additional protection measures to such an installation. With an AC source, I find it hard to believe an electrostatic could build up to begin with. Additionally, enclosures, raceways, and non-current-carrying metal parts are bonded to ground the same whether the system is grounded or not.

 

[mbrooke]

While an electrostatic charge in the kilovolt range is certainly nothing to take lightly, I'd have to believe an ungrounded system posed an imminently greater danger to life and limb before banning or requiring additional protection measures to such an installation. With an AC source, I find it hard to believe an electrostatic could build up to begin with. Additionally, enclosures, raceways, and non-current-carrying metal parts are bonded to ground the same whether the system is grounded or not.

What do you mean by build up though? My understanding is that is either 60Hz voltage or there is not.

 

[Smart $]

What do you mean by build up though? My understanding is that is either 60Hz voltage or there is not.

Sorry. I tend to think of an electrostatic charge as having only a DC voltage component. DC-voltage electrostatic charges can build up over time. There is no law that I'm aware of that says electrostatic charge cannot also have an AC voltage component. But at 60Hz, I do not picture any AC voltage component as building up over time.