Consequences of no Neutral Conductor

[Ingenieur]

That was far from snarky considering you told us electricity could hurt us.

You jumped into this thread by saying ungrounded systems are safer, I disagree with that view.



Those facts do nothing to show ungrounded industrial systems are safer to the personal working around them.

Both are allowed by code

residential is applied for life safety, basically fire protection and shock hazard mitigation

industrial not so much ( or gf protection would be required) but it is getting better
it is designed to minimize risk and prevent an equipment fault from escalating

a delta system is a safer/better choice for some applications
cable handling
all 3 ph loads
appropriate protections

safety is a layer of controls, a series of 'sieves' each catching something different
installation is a small part
engineering
training
Work rules
O & M

my industry requires gf protection on all systems, wye or delta
ngr at 15 A with 6 A relaying
better but not perfect
would not trip 277/1000 or 277 mA
I am involved in writing legislation to require sensitive gf protection
to keep i and t below the Dalziel curve

no one has stated why ug delta is MORE dangerous or less safe
this would imply it is as safe or more safe

applied properly it as safe and in some scenarios more safe
actually less risk
safety level is determined by far more than wye or delta
again lock out/tag out would prevent 90% of injuries/fatalities
build a better mouse trap and along will come a smarter mouse

 

[augie47]

If I understand the situation correctly, a simple fix, if the load allows, would be to move the un-switched lead to the same breaker supplying the switched conductor. You would then simply have one breaker, with neutral from the same panel, feeding a switched and unswitched 14/3.

 

[JFletcher]

If the incoming voltages are not balanced, either from the POCO or voltage drop due to imbalanced 1ph L-L loads, then what happens with 3ph equipment? Just get more current in one winding than the others?

If I understand the situation correctly, a simple fix, if the load allows, would be to move the un-switched lead to the same breaker supplying the switched conductor. You would then simply have one breaker, with neutral from the same panel, feeding a switched and unswitched 14/3.

I think your reply was meant to be on this thread

 

[GoldDigger]

If the incoming voltages are not balanced, either from the POCO or voltage drop due to imbalanced 1ph L-L loads, then what happens with 3ph equipment? Just get more current in one winding than the others?

Yes, and for mostly resistive loads (lighting, heating) it is often not a problem.
For motors the windings seeing the higher voltage will end up carrying higher current, higher even than the simple voltage proportion would lead you to believe. This can cause overheating, and in the case of large current ratios vibration or torque pulsing.
For capacitor input DC power supplies, such as in VFDs, the higher voltage on one phase can cause disproportionately higher current in one set of diodes or SCRs in the bridge, leading to early failure if the equipment is loaded near its design maximum.

 

[JFletcher]

Yes, and for mostly resistive loads (lighting, heating) it is often not a problem.
For motors the windings seeing the higher voltage will end up carrying higher current, higher even than the simple voltage proportion would lead you to believe. This can cause overheating, and in the case of large current ratios vibration or torque pulsing.
For capacitor input DC power supplies, such as in VFDs, the higher voltage on one phase can cause disproportionately higher current in one set of diodes or SCRs in the bridge, leading to early failure if the equipment is loaded near its design maximum.

Thank you. For the bolded part, how would you figure the disproportion, or would it depend on the motor/drive type?

 

[soldier_sss]

Re:

Re:

After investigation, I am of the view with no neutral, the unbalanced current will over heat motors since there is no path for the unbalanced current except through the system.

Also, if the neutral is tied to the earth, then the earth will carry the unbalanced current which means the earth is at a potential and therefore dangerous.

 

[GoldDigger]

Thank you. For the bolded part, how would you figure the disproportion, or would it depend on the motor/drive type?

Yes, it depends.
But each coil will carry current such that I x Z + counter EMF = line voltage. If EMF is the same for all coils at a particular speed, then the current will be proportional to V of that phase minus EMF (all coils have same Z).

This is an oversimplification.

 

[__dan]

After investigation, I am of the view with no neutral, the unbalanced current will over heat motors since there is no path for the unbalanced current except through the system.

Also, if the neutral is tied to the earth, then the earth will carry the unbalanced current which means the earth is at a potential and therefore dangerous.

mmm, no and no.

If the motor sees a line to line voltage imbalance exceeding 1%, the increase in motor losses due to heating can exceed 25% more. The imbalanced voltage at the motor imbalances line current and the resulting rotating magnetic field. See GoldDigger's post above. The voltage imbalance due to any reason, but one of the reasons would be excessive loading and voltage drop on one or two phases and not the other. So, the ungrounded source by itself causes no imbalance.

No, the earth carries no current, at least not as a factor in your power distribution or in a way recognized by code. The ungrounded system should have fully contiguous bonding of all metal, and if the source is floating ungrounded, any connection between the two of any kind is basically a one wire power transmission circuit. Flow would be zero for DC, but for AC, you are charging and discharging a capacitor on every cycle that could be as big as the entire building or all of the facility's metal combined. There will be a flow over the one wire connection which otherwise looks like an open circuit.

The earth reference limits transient overvoltage during hot restriking arcs on an ungrounded system. The solidly grounded system will fault and burn at the location of the fault until the OCPD trips. Localizing or limiting damage to the defective equipment. With no earth reference, transient overvoltage in restriking arcs can cause the system to have insulation failure at basically any point in the system.

The biggest problem with ungrounded systems is that 40 or 50 years down the road, whoever designed, understood, and knew the system is gone. What remains are people who assume the system is grounded, think is was done correctly, was done because that's the way they've seen it done elsewhere, basically a copy or imitation, without understanding the the source is Floating and Not Grounded.

 

[Phil Corso]

Dan...

To be technically correct the capacitor is charged and discharged twice per cycle!

Regards, Phil Corso

 

[mivey]

Well thanks for assuming we are all dolts.

Ummm, I think he was just responding to YOUR post...

 

[iwire]

Ummm, I think he was just responding to YOUR post...

Thanks for bursting my bubble. :rant:


















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