Floating Neutral

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domnic

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Electrical Contractor
If I have a single family dwelling with a 100 amp service with no bonding of any kind, I would have a floating neutral or a 3 phase service with no bonding of any kind. I would have a floating neutral according to the thread on this forum I have just read. What difference would it make to any electrical equipment that I may use?
 
floating neutral

floating neutral

A floating neutral is not solidly connected to ground. Therefore, the voltage on the system wiring can "float" around when referenced to ground. This can be unsafe because during faults or lightning induced transients the system voltage as referenced to ground could become so large that insulation would break down and you might have arcs and sparks.
 
floating neutral

floating neutral

billyzee said:
A floating neutral is not solidly connected to ground.
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how would you solidly connect the neutral to ground in my home 100 amp service ?
 
If by "floating" you mean that the service neutral is not connected to the transformer neutral, then you would get unstable voltages if you have load connected line to neutral. In the case of load only on one leg of a 120/240 volt single phase service, you would get zero volts on the loaded leg and 240 volts on the unloaded leg.
 
jghrist points out the two significantly different usages of 'floating neutral':

If the system neutral is not connected to earth ground, then what you really have is an 'ungrounded system'. The line-line and line-neutral voltages in such a system are correct, but all of these voltages relative to earth and bonded metal are likely wrong. OCPD would not properly function during a ground fault, and external high voltage imposed upon the system (static, transformer leakage, etc) would not be dissipated.

If the neutral bus in the panel is not properly connected back to the transformer neutral, then the line-neutral voltages will be unstable and will depend upon how well the load between the various phases is balanced. In the worst case, the full line-line voltage could be imposed upon circuits that expect the line-neutral voltage.

If the system neutral is not connected to earth ground in your panel, but _is_ connected to earth ground at the transformer, then you have an even worse code violation than the 'ungrounded' system. A fault in such a system could result in significant touch voltages on exposed metal.

-Jon
 
jghrist said:
If by "floating" you mean that the service neutral is not connected to the transformer neutral, then you would get unstable voltages if you have load connected line to neutral.
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That would be an incorrect if not an inaccurate statement. Grounding does not stabilize voltage it only stabilizes the voltage reading between the earth and the energized conductors.

In the case of the grounded conductor that voltage would be 0, and the ungrounded conductors would read the same voltage to ground what would register between the grounded and ungrounded conductors. Voltage reading between the ungrounded conductors would remain the same in either case.
 
weressl said:
That would be an incorrect if not an inaccurate statement. Grounding does not stabilize voltage it only stabilizes the voltage reading between the earth and the energized conductors.

In the case of the grounded conductor that voltage would be 0, and the ungrounded conductors would read the same voltage to ground what would register between the grounded and ungrounded conductors. Voltage reading between the ungrounded conductors would remain the same in either case.
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Sir, I think you may have misread jghrist's post. The quote you grabbed refers ONLY to the neutral being disconnected from the transformer's neutral, which WOULD result in the unstable voltages referenced line-to-neutral. Of course line-to-line would, as you stated, remain stable.

As for line to GROUND (or earth) if the grounded conductor is indeed not bonded, it would be unstable, voltage wise, in both reference to earth and possibly either line conductor (depending on degrees of capacitive coupling perhaps?)

Or I could also be wrong too... :)
 
mxslick said:
Sir, I think you may have misread jghrist's post. The quote you grabbed refers ONLY to the neutral being disconnected from the transformer's neutral, which WOULD result in the unstable voltages referenced line-to-neutral. Of course line-to-line would, as you stated, remain stable.

As for line to GROUND (or earth) if the grounded conductor is indeed not bonded, it would be unstable, voltage wise, in both reference to earth and possibly either line conductor (depending on degrees of capacitive coupling perhaps?)

Or I could also be wrong too... :)
Click to expand...

The neutral refers to grounded conductor. So if there is a grounded neutral at the transfomer and there is a grounded neutral in the panel, but they are not connected to each other, they are still referenced to ground. If the conductor from the panel is NOT connected to ground then it is an ungrounded conductor - not neutral - and indeed would be subject to voltage DIFFERENCE proportionately to the unballanced single phase loading between it and the ungrounded conductors. The voltage would be stable and only vary with the chaning unbalanced loads.
 
floating neutral

floating neutral

if you lose a neutral at the transformer of a 120/240 volt system you have only 240 volts. depending on the loads you have some may be in series and give you an unwanted voltage. if you add up the voltage drops it will be 240 volts.THE TERM FLOATING NEUTRAL IS A OLD WIVES TALE. the term loads in in series would be proper.
 
domnic said:
if you lose a neutral at the transformer of a 120/240 volt system you have only 240 volts. depending on the loads you have some may be in series and give you an unwanted voltage. if you add up the voltage drops it will be 240 volts.THE TERM FLOATING NEUTRAL IS A OLD WIVES TALE. the term loads in in series would be proper.
Click to expand...

I think that we are talking about three different things here.
  1. The star point of Y or the center tap of a coil conductor gets disconnected from the transformer but loads are still connected between it and the ungrounded conductors.
  2. The connection of the grounding electrode to the star point of Y or the center tap of a coil gets disconnected.
  3. Same as 1. but the ground connection to the star point of Y or the center tap of a coil gets disconnected.
 
In residential, The poco usually supplies the transformer, and they usually bond it at said transformer, so unless something happens to the neutral between there and the end use, it is not "floating". One of the reasons we bond and ground everything past that point is help prevent abnormal or accidental voltages from becoming a hazzard. Say if the transformer fails and primary voltages energize the secondary windings at a much higher voltage, or lightning travels down the line, hopefully the grounding electrodes can shunt as much as possible of this energy to ground reducing the amount of damage to the house and equipment.
 
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