Floating Neutral

[din76]

Hi,

I was wondering whether can I get some help on floating neutral.

What does this mean ?

 

[buzzp]

din76, I believe this means the neutral is not grounded.

 

[electricpete]

One way to check - A large voltage difference between neutral and ground with zero current flowing in the neutral would be a pretty good indication of open neutral.

Open neutral means neutral does not have a path back to the source, in which case the neutral node voltage will be determined by "voltage divider" action between the two sets of loads connected to each of the hot legs. Lights will get dimmer or brighter when switching other loads in the house.

 

[din76]

This system we encounter this is a three phase system.So, it could be the neutral not grounded or not done properly !
Thanks for your advise man !

 

[buzzp]

I differentiate between an open neutral and a floating neutral. A floating neutral is just that, there is no connection to ground. It is designed to be a floating neutral. A open neutral is an open on the neutral line, broken wire, etc..

 

[electricpete]

good point buzzp. That distinction not important in the case of 110/220 household power where if there is a neutral it must always be connected (since 110 loads will likely never be balanced with each other).

In three-phase systems it is quite permissible to design for no neutral path between load and source... if the loads are reasonably balanced among the three phases.

 

[busbar]

If you consider neutral being the electrical center (wye or delta) of a 3ø system, yes it is electrically connected to the system-ground electrode Where the neutral (electrical center) drifts due to capacitive reactance or leakage, {no intentional connection between ground and neutral} things can get dangerous; especially where there is neutral shift of a magnitude well exceeding the nominal line voltage.

buzzp’s point of floating vs. open neutral is crucial and may require intervention to limit electric shock or fire.

Without more information it is not possible to advise you, but you may need to get the local electric utility involved in understanding the system at hand. Get help in short order if you do not immediately understand what is happening, what is correctly installed and what is incorrect.

 

[RJF]

"Floating" neutrals is one sense are common in older industrial sites in the US. 480 volt wye systems (powered by something like a 12.47/.48 KV delta wye transformers with no intentional connection of the wye to ground are common where there are three phase motor or transformer loads and no line to neutral loads. The modern equivalent would be the 480 volt high resistace grounded system where a resistor is intentionally placed from the transformer neutral to ground and only three phase or phase to phase loads are supplied. If you have a 3phase 4 wire system where phase to neutral loads are used, the ungrounded phase conductor (neutral wire) needs to be connected to the transformer neutral and then on to ground. I recently ran across a 480 volt system that was used for 277 volt lighting (fed from a 480 delta/480wye isolation transformer off of a large high resistance grounded system) where the connection from the neutral of the transformer was not connected to ground. One of the 480 volt hot phases grounded out and instead of tripping the breaker, the other two hot phases went from 277 volts to ground to 480 volts to ground. An observant electrican noted that he was seeing 480 volts to ground on a supposedly solidly grounded system and investigated. We had to shut the system down to correct the problem.. Ungrounded wye 480 volt systems are still fairly common in industrial systems but you can see some very high and odd voltages if you have hot phase go to ground through an inductance (such as a lighting ballast).

 

[busbar]

Some related info at Thread238-3370 thread238-4513 Thread238-6870 Thread237-7403 thread238-9005 thread238-10998