current on the neutral

[roccovellhung]

Hi all

wonder if someone can help

Checking the load on a 3 phase and neutral supply into a building with a clampmeter and found that there was a small current on the neutral cable, is this normal?

there are 3 phase motors, fleurescents, computer equipment etc within this building - thought that it may be due to harmonics?

any thoughts, advise would be appreciated

thanks

 

[breezer]

will it not be the single phase "kit"?

 

[Big_Spark]

Hi all

wonder if someone can help

Checking the load on a 3 phase and neutral supply into a building with a clampmeter and found that there was a small current on the neutral cable, is this normal?

there are 3 phase motors, fleurescents, computer equipment etc within this building - thought that it may be due to harmonics?

any thoughts, advise would be appreciated

thanks

It will be nothing to do with any of the 400V kit, it will be the Fluorescents and the Electronic Equipement responsible.

What level of current are you measuring?

Please don't take this next question the wrong way, but are you a fully qualified electrician?

The reason I ask is that any I know would KNOW the answer to that question..OK you could have forgotten, we all do, but it is a concern that someone who is not qualified may be interfering with 3 Phase supplies and kit when LEGALLY they must not, let alone common sense.

 

[Millennium_Boy]

There will ALWAYS be some current on the Neutral, for the simple reason that no installation with lights and computers or the like, is likely to be balanced enough to eliminate the neutral current completely.

 

[Big_Spark]

There will ALWAYS be some current on the Neutral, for the simple reason that no installation with lights and computers or the like, is likely to be balanced enough to eliminate the neutral current completely.

Not true.

 

[kendor]

I would hope to see current flowing through the neutral not many flourescent lights or pc's are three phase! many meaning zilch and pc's meaning personal computers as i might be picked up on the fact that some computer suites do tend to be supplied three phase ( Delta to Star through transformer 1:1 ratio ) in order to create a new "clean" earth and neutral)

 

[ban-all-sheds]

You might also be picked up on the fact that some computer suites are supplied with 3-phase power because the equipment inside them actually needs it....

 

[Big_Spark]

OK..engineer hat going on. (Damn, I didn't want to get that hat out of the box!!)

Anyway, on any 3 phase supply you will show small harmonic currents on the neutrals.

Many modern items of office equipment contain switched mode PSU's which can generate a range of harmonic currents. Most of these currents can be disregarded when considering installations, however the magnitude of triple harmonics has been a cause for concern, and raised awareness of the problem to ensure they are allowed for in installation design.

Triple Harmonics effect:

Cable current carrying capacity
Voltage Drop
Overcurrent device operation
UPS, Transformer and Stand-by systems ratings.

For single phase systems the neutral current will normally always equal the phase current so can be ignored.

In 3 phase 50Hz supplies that are balanced, the loads cancel out in the neutral due to the natural consequence of the 120 degree displacement of each phase, however the third, and other triple, harmonics combine to give a neutral current that has a magnitude equal to the sum of the third harmonic of each phase. The heating effect of this neutral current can have a noticeable effect on the temperature at which the cable operates in 4 wire systems.

I'll finish this in a minute...dinner ready

 

[IanDB]

In 3 phase 50Hz supplies that are balanced, the loads cancel out in the neutral due to the natural consequence of the 120 degree displacement of each phase,

. Quite right, however if the phase currents are unbalanced there will be a resultant current in the neutral (remember the vectors).

 

[Big_Spark]

In 3 phase 50Hz supplies that are balanced, the loads cancel out in the neutral due to the natural consequence of the 120 degree displacement of each phase,

. Quite right, however if the phase currents are unbalanced there will be a resultant current in the neutral (remember the vectors).

Ian, I do know, I have not finished writing that post yet, and if you rread the bit under your quote you will see I am fully aware of everything.

 

[roccovellhung]

theres an incoming 3 phase supply but almost all the equipment is single phase - there are a handfull of three phase motors

all lighting uses fleurescent tubes, most of the load is associated with single phase communication equipment (modems, convertors, PA's etc) and PC's - mostly all consisting of internal oscillators - thought that this may be the primary cause.

 

[Big_Spark]

recov, that will actually make it more likely that your loading is unbalanced. Most 3 Phase kit is designed to run balanced, with leading or lagging power factors. A good Installation design will try to balance these leading and lagging PF's as much as is possible, sometimes with the installation of PF correction equipment.

However when you start to load an installation with single phase kit, things tens to get out nof balance pretty quickly.

In a short while I'll finish the post I started yesterday.

One quick way to find out how badly your loading is unbalanced, which will help to identify an unbalanced supply, is to take readings at the source of your supply. You will need a 600A to 1000A rated clamp meter for this, but it is a good start on this path.

Take a reading from each of the three phases, they should be almost identical, but it is likely they are wildly different from what you said originally.

One thing to do is to also take a clamp reading from the main supply neutral and see what loading is being induced by the harmonics across the whole installation.

When you have done that you could then break the installation down into sections, by DB board, and see where the worst part of the unbalanced loading is occuring. If it is the single phase supplies (almost certainly it is) you may want to consider rearranging the circuit layout in the panels to try to balance the loads better. Although be careful that you are not introducing the potential of 400V into offices by having like circuits on seperate phases.

 

[Big_Spark]

Here is the rest of my post about this topic.

Part 1 was

Anyway, on any 3 phase supply you will show small harmonic currents on the neutrals.

Many modern items of office equipment contain switched mode PSU's which can generate a range of harmonic currents. Most of these currents can be disregarded when considering installations, however the magnitude of triple harmonics has been a cause for concern, and raised awareness of the problem to ensure they are allowed for in installation design.

Triple Harmonics effect:

Cable current carrying capacity
Voltage Drop
Overcurrent device operation
UPS, Transformer and Stand-by systems ratings.

For single phase systems the neutral current will normally always equal the phase current so can be ignored.

In 3 phase 50Hz supplies that are balanced, the loads cancel out in the neutral due to the natural consequence of the 120 degree displacement of each phase, however the third, and other triple, harmonics combine to give a neutral current that has a magnitude equal to the sum of the third harmonic of each phase. The heating effect of this neutral current can have a noticeable effect on the temperature at which the cable operates in 4 wire systems.

The current rating of cables found in BS7671 and related standards assumes that at full load the neutral current will be practically zero. Whilst in practise the current is very rarely zero on standard 50Hz supplies, this is simply an indication of an out of balance loading of the installation. Assuming the phase conductors are not overloaded, then neither will the neutral conductor.

In most current rating tables the rating of 4 core cables is the same as that of 3 core cables, but theoretically the 4 core cable should be given a higher current rating. This additional capacity of a 4 core cable can offset, to some extent, the heating effects of the harmonic currents induced into the neutral conductor.

For this overload protection is only provided if;

Iz = It Ca Cg Ci Ct >In > Ib

Where

Iz is the current carrying capacity of the cable for this particular installation
It is the tabulated current carrying capacity of the cable
Ca,Cg,Ci,Ct are enviromental and installation correction factors
In is the nominal current of the overload protective device on the phase conductors
Ib is the load design current

If third harmonics are present then the formula holds true for 4 core cables where the triple harmonic content of each phase does not exceed 15% of the fundermental, for greater harmonic content, allowances must be taken into account.

Triple Harmonics

Triple harmonics are multiples of the 3rd harmonic, ie 3rd, 9th, 12th, 15th etc. The third harmonic will oscillate at 3 x 50 Hz or 150 Hz. These harmonics are drawn by power supplies to computers, printers, other office electronics and certain types of lighting equipment.

Due to their nature, triple harmonics and multiples of the triple harmonic are not cancelled out in the neutral, they actually add together. For a load that is pure thrid harmonic, the neutral current (Ibn) is actually 3 times the phase current (Ibph). This could and does lead to overheating of cables and equipment, increases the voltage drop of the cable and ultimately degrades the current carrying capacity of the cable.

When designing installations, the designer needs to make allowances for the third harmonic content, both in cable sizes and in switchgear, along with overcurrent protection to match.

The Neutral Current(Ibn) can be easily calculated if the third harmonic current is known.

Ibn = 3h / 100 x Ibph

Where:

Ibn = neutral current
Ibph = phase current
h = triple harmonic as a percentage of the phase current.


OK, so I could bang on about this for hours, and I have better things to do, but you can see that harmonic current s are a problem that requires addressing, usually at the design stage of an installation, however it is also something that needs to be seriously looked at when addressing unbalanced loading issues in existing installations.

As electricians we are taught that Neutral = Phase current = zero, that is not true, single phase supplies attatched to unbalanced 3 phase installations can have triple harmonic currents imposed upon them, having a negative effect on the equipment within the installation and th infra-structure of that installation.

Have any of you actually wondered why the neutral is normally the one that suffers heat stress in badly loaded or installed systems? Well now you know what the reason is...outside of loose terminals anyway.

 

[kendor]

You might also be picked up on the fact that some computer suites are supplied with 3-phase power because the equipment inside them actually needs it....

go back read again, i said you get computer suites supplied three phase! the equipment also needs single phase supplies aswell hence the neutral.

I would have thought it obvious from the original post and the author later confirms the existance of single phase equipment connected to the supply, whithout resorting to a scientific explanation about harmonics etc it is elementary stuff to realise that with single phase equipment connected to a three phase and neutral supply that there will be current flow through the neutral, in a "perfectly" balanced three phase system with just 3 phase equipment connected then no current agreed but we are not talking hypothetical instances here but the real world, how many installations in the country have "perfectly" balanced supplies for a start, it cannot be achieved, the designer of a system will try to get as close as possible.
But as stated before the original question did mention flourescents computers etc.
Another point, there are few exclusively three phase installations as you are always going to need lights, and power.

 

[breezer]

what is wrong with "balancing the single phase load across all 3 phases?

for sake of argument 300 lights, 100 on each phase (and obviously all connecting to neutral) = balanced load (almost)