Neutral in a three phase system - Page 1
Forum section: electronics / general

author: Dellyjoe
author_rank: Regular Contributor
timestamp: October 07, 2020, 11:05:33 pm
content: Hello everyone,I might have a simple question to answer but I'm having a little hard time understanding an answer to my question.My question is: assume you have a three phase system lets say a generator, (3 hot 1 neutral) wire. I understand Neutral completes the circuit for each of the hot wires and if that is the case does the neutral wire carry three times the amount of current on it?In some circuits I see the neutral conductor is larger implying that it does, and in some circuits it is the same size, which means maybe the hot wires are oversized. If you look at a single phase circuit in your house they are the same size b/c each both are carrying the same amount of current, but in a two phase circuit like a washer, what size in the neutral? the same as one hot wire? I know they up the size of the conductors to 10 from 12, and is this because it is a two phase system and the neutral needs to carry two times the amount of current, or is the system just producing more power?Let me know what you think and thank you for reading,Joe

author: themadhippy
author_rank: Super Contributor
timestamp: October 08, 2020, 01:00:40 am
content: In the ideal world were each phase is drawing the same current (balanced) the neutral current will be zero,once the phase aren't equal (unbalanced) the "extra" current has to go somewhere,and that's the job of the neutral.You could do some long sums to calculate the neutral currents,but a pictures easier,say youve 100A on 1 phase ,50A on phase 2 and 130A on phase 3 .Draw a line 100 units long to represent phase 1. From the end of this line, measure 120 degrees clockwise and draw a line 50 units long in this direction to represent phase 2. From the end of this line, measure 120 degrees clockwise and draw a line in this direction 130 units long to represent phase 3. Measure the distance between where your phase 1 line started and your phase 3 line ended. That lines your neutral current,in this case 70 units long,or 70A

author: IanB
author_rank: Super Contributor
timestamp: October 08, 2020, 01:18:32 am
content: Quote from: Dellyjoe on October 07, 2020, 11:05:33 pmIt's not a matter of opinion, it is a matter of fact.As mentioned above, in an ideal situation with a perfectly balanced load on each phase the neutral wire carries no current.However, that doesn't mean the neutral wire can be eliminated as it is the connection that preserves a proper voltage on each phase.If the loads are not balanced, the neutral wire will be required to carry the differential current that results from the imbalance.It is an engineering calculation to decide how big to make the neutral wire. However, a good rule of thumb would make it the same size and the line, or phase, conductors.

author: Dellyjoe
author_rank: Regular Contributor
timestamp: October 08, 2020, 02:26:28 am
content: Quote from: themadhippy on October 08, 2020, 01:00:40 amThank you Themadhippy, that makes sense after seeing a three phase gen source output voltage plot.

author: Dellyjoe
author_rank: Regular Contributor
timestamp: October 08, 2020, 02:27:37 am
content: Quote from: IanB on October 08, 2020, 01:18:32 amThank you IanB for answering. This really help me clear it up. and to confirm in a single phase circuit it would carry the same much current as what would be in the Hot Line.

author: stefan_trekkie
author_rank: Contributor
timestamp: October 08, 2020, 11:46:42 am
content: In most arias the household electrical supply is 3 phase .. One house or apartment is supplied with one phase, the next one is with second phase .. The third one.. The neutral in that case is coupled together on some point and as explain above - zero potential on that spot on ideal loads.Most direct 3 phase loads (motors, heating etc) do not use neutral for supply but only for protection on the metal cases and the load can be with triangular or star coupling

author: Circlotron
author_rank: Super Contributor
timestamp: October 08, 2020, 12:15:47 pm
content: Quote from: themadhippy on October 08, 2020, 01:00:40 amYep, provided also the three currents are sinusoidal. If there are harmonics present in the phase currents the neutral wire can cop a beating even though the RMS value of the phase currents may be equal.

author: themadhippy
author_rank: Super Contributor
timestamp: October 08, 2020, 12:56:54 pm
content: Quoteindeed,and why the neutral conductor can be larger than the phase,but now were going well beyond beginners levelQuoteremind me never to touch anything metallic that you've wired.

author: stefan_trekkie
author_rank: Contributor
timestamp: October 08, 2020, 03:52:10 pm
content: Quote from: themadhippy on October 08, 2020, 12:56:54 pmLike this one ?or the other little one (below)

author: langwadt
author_rank: Super Contributor
timestamp: October 08, 2020, 04:04:51 pm
content: Quote from: stefan_trekkie on October 08, 2020, 11:46:42 amhere everyone gets all three phases. Usually 3-phase outlet is available for stoves, washing machines and water heaters, lighting and outlets for different rooms distributed on different phases

author: sibeen
author_rank: Frequent Contributor
timestamp: October 08, 2020, 10:42:16 pm
content: Quote from: Circlotron on October 08, 2020, 12:15:47 pmWhilst not so much of an issue now days this was certainly a problem in the past when power supplies regularly drew highly distorted currents. The third harmonic generally dominates in single phase loads with harmonic rich input currents and the third harmonics add up in the neutral when you have single phase loads spread across a three phase system. This was such an issue that 20 and 30 years ago it was quite common to see designs for data centres specify that the neutral be twice the size of the phase cabling. In a worst case condition the third harmonics could add to a total of 1.73 {sqrt (3)} times the phase current in a balanced three phase system.Another regular feature in data centres was the use of triplen transformers which stopped the flow of triplen harmonics back to the source. It's not only the third harmonic that adds in the neutral, although generally that by far dominates, but all the odd multiples of three; so the third, ninth, fifteenth, twenty first etc. The triplen transformers would be located close to the load and the triplen harmonics could flow between the output of the transformer and the load but be stopped at the transformer and not flow back to the source. In places like the USA where the voltage was generally distributed at 450 volts through the centre a power distribution unit (PDU) would be used close to the load to drop the voltage back down and in this case the transformer was also used as a triplen transformer. In areas like Australia where a PDU wasn't required for stepping down a voltage a triplen transformer was often specified.

author: DenzilPenberthy
author_rank: Frequent Contributor
timestamp: October 09, 2020, 10:06:18 am
content: Quote from: stefan_trekkie on October 08, 2020, 03:52:10 pmYes. combined neutral and earth is forbidden here in the UK (except in the supplier's distribution network)

author: ahbushnell
author_rank: Frequent Contributor
timestamp: October 09, 2020, 04:14:33 pm
content: There are two types of three phase systems. One is delta and the other Y. The Y configuration uses a neutral. Normally its tied to ground at one point near the transformer. The delta does not have a neutral.https://en.wikipedia.org/wiki/Three-phase_electric_power

author: Cerebus
author_rank: Super Contributor
timestamp: October 10, 2020, 01:32:13 am
content: Quote from: DenzilPenberthy on October 09, 2020, 10:06:18 amThe vast majority of of UK installations are TN-C-S (PME), with a combined protective earth and neutral (PEN) running between served premises and the local substation, connected additionally to earth at various points on its way, the premises protective earth is simply another connection to neutral at the incomer.There are also comparatively rare TN-S supplies (usually older or very large supplies) and even rarer TT supplies (almost exclusively in rural areas involving overhead supplies where guaranteeing continuity of the supply earth path is difficult) and extremely rare TN-C (needing an explicit exemption from the Supply Regulations) and almost non-existent IT supplies (forbidden from use in the pubic supply of electricity, some private systems exist).

author: stefan_trekkie
author_rank: Contributor
timestamp: October 11, 2020, 08:44:34 pm
content: Quote from: ahbushnell on October 09, 2020, 04:14:33 pmIt may uses but in most cases do not uses the neutral at the Y point. I have worked on thousands of small to heavy factory industrial machines and never saw a neutral on Y point. That been mostly 3 phase asynchronous motors, some heaters, inductors and transformers. .. That been of USSR, Germany, Czech republic, Hungary, Japan and Bulgarian made machines.It looks like this:That is Δ or Y, no neutralQuote from: ahbushnell on October 09, 2020, 04:14:33 pmDelta uses artificial neutral if neededQuote from: DenzilPenberthy on October 09, 2020, 10:06:18 amAs this forum is with people all over the world, one thing is clear .. Many places and many standards exist

author: elekorsi
author_rank: Regular Contributor
timestamp: October 12, 2020, 07:22:22 am
content: Hi all, my first post here, well you have to start somewhere, rightI am from Europe, so we are using IEC standards, would be interesting to hear more from people across the ocean.In industrial applications most common system is TN-C, with three phase conductors + combined protective earth and neutral, hence the name of that conductor is PEN. The minimum cross section for PEN is 10mm2 Cu or 16mm2 Al. (8 AWG and 6 AWG if i have correct cross tables). Where the cross section is smaller than 10mm2 you must make an TN-S system or TN-C-S. In the electric cabinet you usually put two bars, one for PE and one for N. With the PEN conductor you ALWAYS first connect to the PE bar and then make a bridge to the N bar. After this separation it is prohibited to combine PE and N anywhere in the system. It is also prohibited to disconnect the PEN anywhere on its path, for example with the switch or circuit breaker (you must use 3 pole switch).We always tend to make the current flow in the PEN as close to zero as possible (it is depending on the equipment you are powering). If you have mostly 3 phase equipment (with symmetrical phase loads, like motors, heaters, etc.) the current through PEN will be 0 or close to that.The IEC standard states that the PEN conductor can be half the size of current phase conductors when phase conductors are larger than 35mm2. Of course you do this when you have mostly symmetrical consumption on the equipment. For example it is quite common to put lets say 3x240mm2 + 1x120mm2 for the power supply of one cabinet. Not long ago we were connecting a CAT diesel generator rated at 630kVA, we used 6x300mm2 for the phases and 1x300mm2 for the PEN.Of course everything must be engineered (calculated). If you have problems with harmonics there could also be need for oversized neutral conductor...About the residential power distribution, here mostly every house (95%) are connected to three phase system, with the TN-C-S system being most common. I have 3x25A main fuses in the metering cabinet, this gives us 17,3kVA calculated power.

author: f4eru
author_rank: Super Contributor
timestamp: October 12, 2020, 08:36:22 am
content: Some countries in Europe have 3-phase supply, even for small apartments, for example Germany. In that case, each owner has to assign each 1phase load to a phase, an balance approximately his system.Many countries have single phase connection for residential, for example France has single phase as standard under 63A. You pay much more for 3-phase.Different houses are then connected to a different phase, to balance out at the street level, and transformer are always 3-phase and huge.Only the North America and Japan have the "interesting" split phase 180° system, because the transformer are single phase, and are much more numerous and therefore smaller because the 120V wiring at street level has to be shorter (4 times more copper needed at 120V than 240V for a given length, power, and line efficiency !!)

author: Alti
author_rank: Frequent Contributor
timestamp: October 12, 2020, 11:02:56 am
content: Quote from: Dellyjoe on October 07, 2020, 11:05:33 pmThe cross section of the conductor is not only determined by the current load.There are several criteria that are considered.Quite contrary to popular beliefs, nominal current load capability is not on the entry of that list, not even in the first half.Some earthing systems also use what is called PEN which is a combined protective earth and neutral in one wire.This is NOT a neutral wire.The lack of understanding the difference in between N, PE or PEN is life-treatening.The topic is "Neutral in a three phase system", not PEN.Quote from: stefan_trekkie on October 08, 2020, 11:46:42 amI am absolutely sure that there is not a single load that has neutral connected to a metal case for protection.Quote from: langwadt on October 08, 2020, 04:04:51 pmI have never seen a household washing machine that is powered with three phase AC.Do you have 3-phase receptacles in bathrooms in DK?

author: langwadt
author_rank: Super Contributor
timestamp: October 12, 2020, 12:13:51 pm
content: Quote from: Alti on October 12, 2020, 11:02:56 amnot a receptacle, usually five terminals behind a cover like this:https://www.wattoo.dk/media/catalog/product/cache/1/image/1200x/040ec09b1e35df139433887a97daa66f/6/f/6fe2181e-0906-4f3e-a133-73b29e00b2eb.jpg_5.jpgso you can put washer and dryer on separate phases

author: stefan_trekkie
author_rank: Contributor
timestamp: October 12, 2020, 12:19:27 pm
content: Quote from: Alti on October 12, 2020, 11:02:56 amIt even has a name/type ... Type II class of electrical protection.Pic below ..

author: DenzilPenberthy
author_rank: Frequent Contributor
timestamp: October 12, 2020, 12:58:54 pm
content: Quote from: stefan_trekkie on October 12, 2020, 12:19:27 pmI think you mean Class I? Class II is double insulated and requires no earth connection.https://en.wikipedia.org/wiki/Appliance_classesIn any case, the system in this diagram which uses a combined Neutral and Earth conductor is not safe and is forbidden in many countries. (Certainly in the UK, I can't speak for other places.)

author: richard.cs
author_rank: Super Contributor
timestamp: October 12, 2020, 01:16:51 pm
content: Quote from: Cerebus on October 10, 2020, 01:32:13 amI am not sure I would call either of those rare in the UK, though it is very dependent what part of the country you are in. TN-S is very common in many towns using the lead sheath of PILC cables as the separate PE connection. Yes it is slowly being converted to TNC-S in many cases (often silently in the street so it's often completely unclear what a given consumer has) but there is a lot left. In addition to rural areas, TT is mandatory in building site, marinas, caravan sites, etc. and is somewhat common in suburban areas too.We also have a bit of PNB, which is kind of halfway between TNC-S and TNS and generally poorly understood by a lot of people. It moves the neutral-earth bond into the consumer installation, giving them the features of a TNS supply (for a single customer it is basically identical), but people often tend to group it with TNC-S even though the failure modes are totally different and more TNS-like.

author: themadhippy
author_rank: Super Contributor
timestamp: October 12, 2020, 01:44:56 pm
content: QuoteThats the rare,but still out there TNC supply method,not banned,but has plenty of regulations regarding its use,including i believe, getting permission from some government department

author: Alti
author_rank: Frequent Contributor
timestamp: October 12, 2020, 01:50:38 pm
content: Quote from: stefan_trekkie on October 12, 2020, 12:19:27 pmThis looks like TN-C, it does not expose neutral. The Q wire is called PEN.This is a type of earthing system that was quite popular in cities long time ago when wire cost was an issue. Very dangerous, in case of any PEN damage the user receives full phase voltage via "metal case". No RCD possible, no fuse will trip, no warning, just full zap.Quote from: langwadt on October 08, 2020, 04:04:51 pmSo the "washer and dryer" is one appliance?In a similar maner as kitchen stoves are?My kitchen stove is a set of 5 single-phase loads (4xrings + 1xgrill) and in the terminal box at its back you can configure/distribute the load to be connected into 1,2 or 3 phase supply.Is this similar or are these "washer and dryer" totaly separate appliances?I could understand tying an appliance to the wall if it is 10kW but a washing machine is 2kW (9A) so I am wondering why not using two receptacles on individual circuit breakers?Maybe this comes from some weird DK legislation.

author: elekorsi
author_rank: Regular Contributor
timestamp: October 12, 2020, 01:52:45 pm
content: Quote from: stefan_trekkie on October 12, 2020, 12:19:27 pmThis is prohibited here from late 80s, early 90s for residential buildings. I would assume that Bulgaria uses IEC standards, since it is in EU. As i said before, usually you will only meet the TN-C system with the PEN conductor at larger loads, since the minimum cross section for PEN is 10mm2 Cu.

author: Cerebus
author_rank: Super Contributor
timestamp: October 12, 2020, 02:07:22 pm
content: Quote from: richard.cs on October 12, 2020, 01:16:51 pmI didn't say "rare" I said "comparatively rare". Not the same thing as saying "rare".

author: madires
author_rank: Super Contributor
timestamp: October 12, 2020, 02:26:25 pm
content: Quote from: ahbushnell on October 09, 2020, 04:14:33 pmIt would be better to say that there are two ways to wire 3-phase loads. The 3-phase system stays the same for both wiring methods. The point of the delta circuit is to have a higher voltage created by the 120° phase shift. Some machines have a large switch to change the circuit from Y/star to delta and vice versa. The motor will be powered up in Y/star mode and then switched to delta mode for maximum power (a kind of soft start).

author: Alti
author_rank: Frequent Contributor
timestamp: October 12, 2020, 03:31:36 pm
content: Quote from: madires on October 12, 2020, 02:26:25 pmI think there are may more than two. Even if transformer is wye wired then just three wires might be exposed with whole transformer floating. Quite exotic but possible. I bet one could also run three phase system on two wires and run third phase through earth but that could be quite unreliable, considering that earth at most locations is not a great conductor. Or running a wye transformer with centroal point grounded but not exposed.

author: richard.cs
author_rank: Super Contributor
timestamp: October 12, 2020, 03:50:38 pm
content: Quote from: Alti on October 12, 2020, 03:31:36 pmOh yes there are many. Generally we can blame most of them on the Americans*- things like corner-grounded delta and high-leg delta (the neutral is a centre tap on one of the delta windings). So you don't just have the four combinations of source/load Y/delta, but also which if any node is earth-referenced, whether that is a low or high impedance connection, and whether that is distributed or not. Yey, standards.* As a result of having standardised very early on bipolar DC, and consequently on split-phase, and also on awkwardly low utilisation voltages, they have all kinds of exciting bodges to combine split phase with three phase and generally have more voltage and phase combinations than Europe. 120/208/240/277/480 V.

author: eeviking
author_rank: Regular Contributor
timestamp: October 13, 2020, 06:50:15 pm
content: Quote from: Alti on October 12, 2020, 11:02:56 am3-phase (380V) washing machines where normal here up to the 90s. Our 1-phase outlets where only 10A 220V and could not power big appliances.Today the 3-phase circuits for washer/dryer is a legacy thing, as all newer installations have 13A 230V 1-phase outlets.3-phase is still used for stoves, ovens, water heaters etc.All homes have 3-phase 400V/230V supply (normally 3x35A in newer homes), RCDs are mandatory and TT earthing is used almost everywhere.

author: Galenbo
author_rank: Super Contributor
timestamp: October 14, 2020, 07:50:37 am
content: Like everybody said: There is no current in the Neutral when the load is balanced......at 50Hz (60HZ USA)Some (balanced) loads cause harmonics and bad power quality.The third harmonic (150Hz/180Hz) adds up in the Neutral wire, instead of cancelling out.This sometimes blows (or heats) up that cable, without being seen by ordinary indicators.First-page search result on gooooglehttp://www.marcspages.co.uk/pq/3240.htm

author: Alti
author_rank: Frequent Contributor
timestamp: October 14, 2020, 11:54:12 am
content: Quote from: Galenbo on October 14, 2020, 07:50:37 amIt is also worth to note that neutral wire has some nasty properties that phase wires do not posess and because of that it deserves additinal attention.In earthing systems whereRCDsare possible a phase wire leakage can be easily isolated and diagnostics is quite simple, even by an unskilled user by just clicking breakers on distribution board, plugging and unplugging appliances. Fixing that is another story but after you isolate the leakage you still have the power in the rest of the house/circuit. However, with neutral wire leakage and single pole circuit breakers this dance becomes super complicated, including digging deep in distribution board, unscrewing wires, messy job. Even when you turn off all individual circuit breakers, even when you unplug all the appliances, the RCD will trip in this case so you are screwed if that is Friday evening.Another nasty property of neutral is that if it gets broken (like interrupted, disconnected, unscrewed etc) before or at separation into three neurtals, then single phase devices will most likely get close to 400V instead of regular 230V. Since most devices start with rectifier nowadays, the input cap will get 600V intead of 350V and. It will trigger all unfortunate events like TVSes start to burn, caps explode, possibly with fire. Even when the kill is not instantaneous, it will bite eventually after time. Not a pleasant thing to happen. One might think "why to disconnect neutral at all". Well the problem is that neurtal might get damaged (excavations, overhead line), also has to be disconnected in many situations, like in the one above. Also, 3-phase RCDs disconnect neutral and although they should obey the sequence of disconnection and connection, this might become a problem with low quality or worn out gear. Then, when doing any maintenance work, it is mandatory to isolate all working wires (including neutral) so the isolators are placed along the way.These two usually do not necessitate thicker gauge but it is better to keep an eye on neutral.

author: madires
author_rank: Super Contributor
timestamp: October 14, 2020, 02:18:11 pm
content: Wouldn't a single phase device become an open circuit when the neutral wire is broken?

author: wraper
author_rank: Unknown
timestamp: October 14, 2020, 02:32:49 pm
content: Quote from: madires on October 14, 2020, 02:18:11 pmOnly if neutral wire is broken after a point where no any other devices are connected. Otherwise phase with lower load will get overvolted (or 2 phases if one phase has much higher load compared to others) as current will pass to neutral through other equipment.

author: SeanB
author_rank: Super Contributor
timestamp: October 14, 2020, 05:42:55 pm
content: Had loss of neutral a few times, from faulty connections in the supply side, to the last few being caused by the copper thieves stealing all the neutral copper they could reach before it went underground in the substation. I turned off my supply when I saw the mains was over 260VAC, and then measured the meter supply voltages and saw the varying voltages, so turned off the incoming supply and phoned the metro to report the loss of neutral in the line.Not much damage by me, just one VDR unit that sacrificed itself to protect, but not good for those who were left with supply till the electricians came out and turned power off at the substation, they had a lot of equipment destroyed as various geysers cycled on and off heating up water, driving phase voltages up on the other phases. Coffee shop on the corner lost ovens, point of sale and lighting to the surge, as the various phases went up to near 300 VAC in turn.

author: Alti
author_rank: Frequent Contributor
timestamp: October 15, 2020, 12:25:05 pm
content: Quote from: SeanB on October 14, 2020, 05:42:55 pmThat is most likely a consequence of tradition in electrical engineering.I am afraid the neutral connections were not faulty at all.These were most likely same as phase connections.The assumption that all three phases are equally loaded is difficult to meet. Especially when you have uncorrelated single-phase loads, like residential. This is not about harmonics but due to a fact cabling is protected with thermal breakers. Everyone knows C32 can give 60A for a minute, right? So some guy turns 60A for a minute, heating up L1 and N and once finished, his neighbor loads L2 and N again and afterwards third neighbor takes shower on L3 and N and then the round robin starts from the beginning. Of course phase wires have (adequate for phase wires only) thermal protection, but not for neutral wire. Abuse neutral 1000x and then you have 300VAC on a VDR. The line was designed for twenty years but died after six.Of course I do not know what had been the cause there but the fact is that in era of fuses it was darn difficult to protect neutral from overloads so it has been decided it is "better" to allow neutrals without any protection. You cannot just stick a fuse in neutral path, can you? Nowadays in era of circuit breakers guess how upgrades of existing installations are made? Since in the past this risky business was working mostly fine, they just replace fuse for circuit breaker even though they could protect neutral easily as there are circuit breakers that protect four wires. The downside is that protecting neutral would mean more servicing calls caused by overloading, earlier upgrades - nobody likes that hassle, espeically the decision maker.Electrical codes are still aligned with tradition and allow installing a cable that is without overload protection.

author: Cerebus
author_rank: Super Contributor
timestamp: October 15, 2020, 02:28:58 pm
content: Quote from: Alti on October 15, 2020, 12:25:05 pmHad you bothered to read another few words before correcting Sean:Quote from: SeanB on October 14, 2020, 05:42:55 pmI think "Not being there because some bugger's stolen it" counts as a faulty neutral.

author: Alti
author_rank: Frequent Contributor
timestamp: October 15, 2020, 02:49:26 pm
content: Quote from: Cerebus on October 15, 2020, 02:28:58 pmThat is how I understood SeanB:Code:"N times loss of neutral" and"K times fault connections, K<N" and"P-th to N-th time was caused by thieves, P<N"more or less.You mean whether K+P=N or K+P<N actually? I must admit I have no idea what your remark means.Yes, I did read whole post from SeanB.My remark applies to K cases SeanB mentioned.Not to N-P cases where thieves stole the cable.Not to all N cases of lost neutral where various factors had influence.

author: Cerebus
author_rank: Super Contributor
timestamp: October 15, 2020, 03:22:14 pm
content: Forget about it, I have a feeling that further explanation would be neither enlightening nor edifying.