Loss of Neutral in Mains supply system - Page 1
Forum section: electronics / general

author: opampsmoker
author_rank: Regular Contributor
timestamp: December 09, 2020, 01:00:56 am
content: Hi,Is "loss of neutral" more of a problem for equipment supplied by a three phase supply, or more of a problem for equipment supplied by just a Line and Neutral ?(single phase supply).That is, a Line and neutral that has obviouly come from a three phase supply (since all single phase mains ultimately comes from a three phase supply somewhere)."Loss of neutral" makes The Line to neutral voltage go up to 587Vpk in UK. I am amazed that all things designed to be connected to the mains dont have some kind of protection so that they dont blow up when "Loss of neutral" occurs?

author: langwadt
author_rank: Super Contributor
timestamp: December 09, 2020, 01:04:53 am
content: Quote from: opampsmoker on December 09, 2020, 01:00:56 amthe voltage will depend on the load on each phase, if the load on all phases are the same you don't need a neutral

author: Berni
author_rank: Super Contributor
timestamp: December 09, 2020, 06:35:56 am
content: What happens when neutral is lost is entirely dependent on what loads are connected at that time.If you happen to be on the most loaded phase the voltage drops, if in the least loaded phase it goes up. It typically does not cause the worst case between phase voltage to appear, but generally enough to blow stuff up. Its unreasonable to design everything to handle pretty much double the input voltage.As for 3 phase loads. They will indeed also be upset if they have a neutral connection. Things like 3 phase motors typically won't care because they typically don't have a neutral connection at all (Even in Y mode they usually don't because the Y makes its own neutral), but the 3 phase loads that are actually a collection of single phase loads glued together will care. Things like the kitchen cooker or oven that have multiple heating elements distributed between the multiple phases will see more voltage across some heating elements.In any case loosing neutral on the main feed to the building is a very bad day. But things like industry typically have there own on site transformer.

author: opampsmoker
author_rank: Regular Contributor
timestamp: December 09, 2020, 11:13:36 am
content: QuoteThanks, even if single phase?.....and in that case, is that because earth acts as the return instead of neutral?Its hard to see the overall schematic of whats going on when a neutral is lost.Ive had contact with a customer who has declared that they will not use loads of isolated SMPS's running of the phases of a three phase input, but will instead use a huge three phase mains transformer, and the reason because it means more resilience to "loss of neutral".Is this valid?

author: Berni
author_rank: Super Contributor
timestamp: December 09, 2020, 12:44:23 pm
content: With single phase you are safe, loosing neutral there just breaks all return current from live and so the voltage across the terminals drops to 0V.The reason this is a problem with 3 phase is that a heavy load on one phase is so low resistance that once neutral gets disconnected the voltage on neutral becomes almost equal to the other phase. As a result a heavy load on L1 will make the voltage on neutral be the same as L1. So when you also have a load connected between L2 and N that load is suddenly seeing almost as much voltage as if it was connected between L2 and L1, making it see almost double the voltage.Using a 3 phase transformer to avoid this problem is however silly. Just like 3 phase transformers can be connected in the delta configurations so can switching PSUs. You simply buy a 400VAC input PSU and connect it between L1 and L2. That way the power supply is constantly seeing the higher inter-phase voltage, but is designed to run on it. If you require the PSU to keep working if one of the phases disappears (just as likely as neutral disapearing) then a switchmode supply with a 3 phase input rectifier can be used. This will make the PSU keep running if at least 2 phases are still alive (otherwise the PSU simply does not run, does not get damaged). This loss of 1 phase out of 3 phases however can have catastrophic consequences for 3 phase motors since the motors keep running but with 2 of the 3 windings overloaded, if they are kept running at rated power means that they overheat and die.EDIT: Actually for motors its 1 winding out of 2 to be overloaded. Since motors typically run in delta means that loss of a phase turns it into essentially a single phase motor with the according reduction of max power.

author: Alti
author_rank: Frequent Contributor
timestamp: December 09, 2020, 12:50:05 pm
content: Quote from: opampsmoker on December 09, 2020, 11:13:36 amSimilar discussion here.A "loss of neutral" typically means a break in neutral conductor upstream of a point where it splits (into three neutrals).

author: David Hess
author_rank: Super Contributor
timestamp: December 09, 2020, 05:33:22 pm
content: Quote from: Berni on December 09, 2020, 12:44:23 pmSplit-phase systems are still single-phase but they include a neutral, and loss of the neutral in a split phase system results in the same problems that loss of neutral in a 3-phase system causes. I actually had this happen to me many years ago when a tree fell on the line feeding my house breaking only the neutral. I temporarily connected a big 240 volt transformer with the center tap connected to neutral to force balance until the line could be repaired. A big 3-phase Y transformer could be used to do the same thing for 3-phase.

author: opampsmoker
author_rank: Regular Contributor
timestamp: December 10, 2020, 12:48:29 am
content: Thanks, from what kind contributors have said, it looks like single phase supplied equipment is still at risk, because if the neutral that runs with the three phases from which the single phase was hived off gets disconnected , then the overvoltage on the single phase will occur.I am just baffled that no offtheshelf offline power supplies have some protection from this overvoltage.....even if it was just some circuit to switch it off from the mains in the event of a high voltage due to "Loss of neutral".

author: Berni
author_rank: Super Contributor
timestamp: December 10, 2020, 06:58:29 am
content: Quote from: David Hess on December 09, 2020, 05:33:22 pmWith single phase i meant just single phase where you only have L and N coming in. We don't have split phase here in Europe. That split phase indeed has the same problem as 3 phase when it comes to neutral loss.Luckily we never got a neutral fault, but we did lose a phase before due to a old wobbly power pole working the connection loose. I simply disconnected that phase and bridged it to another one for until they came to fix it. But i suppose one possible fix would also be to plug in a 3 phase motor and leave it running to make a "2 phase to 3 phase rotary converter"Quote from: opampsmoker on December 10, 2020, 12:48:29 amIt requires the PSU to be overspecced and that costs money. Theoretically this would also mean your TV could not use the regular 220V single phase plug, but would instead need the appropriate 3 phase plug or have it wired in directly by an electrician such as an HVAC unit. The regular plugs are not rated for 400V interphase voltage.In reality a properly designed PSU does have an input fuse and a MOV on the input, so sending 400VAC into it would just blow the fuse and possibly damage the MOV. Loss of neutral happens so rarely that it is not worth overdesinging every PSU for it. Lightning strikes are more common and even those have a tendency to blow switching supplies here and there. You could overdesign for that too, but up to what point? There are tiny lighting surges that raise the voltage by 10s of volts and there are proper direct strikes that raise the voltage to 10s of kV

author: opampsmoker
author_rank: Regular Contributor
timestamp: December 10, 2020, 12:33:29 pm
content: HiThere is a device called the AVS30 by Sollatek, which purports to be able to switch off the supply if an overvoltage caused by loss of neutral occurs.AVS30https://docs.rs-online.com/0164/0900766b8002a701.pdf..However, the spec sheet says that the response time to overvoltage is 20ms……….which seems to be of no use whatsoever..since as you can see from the attached LTspice simulation, the overvoltage of up to 587Vpk can occur within a microsecond. So why is the AVS30 even on sale?Its response time is way to slow….your products would have blown up.AVS30 datasheethttps://www.sollatek.com/wp-content/uploads/2015/08/AVS30-data-sheet.pdf

author: Gyro
author_rank: Super Contributor
timestamp: December 10, 2020, 08:52:39 pm
content: PEN (Protective Earth and Neutral) fault detectors / protectors have become a key item for EV charging installations. Probably worth some investigation.

author: Simon
author_rank: Unknown
timestamp: December 11, 2020, 12:55:22 pm
content: Quote from: opampsmoker on December 09, 2020, 01:00:56 amIf you remember a few years ago a street in the UK ended up with much more than mains voltage and pretty much every appliance device and light bulb blew up. I think the cause was narrowed down to the phase breaking and that street got 400V instead of 250V

author: dmills
author_rank: Super Contributor
timestamp: December 14, 2020, 11:51:11 pm
content: Over voltage relays and shunt trip breakers are a thing, but it is a rare enough event that for the most part this is why you have insurance.It is however a somewhat popular hobby in the live events production (remember when we had live events?) scene, where 100kVA and up diesel sets outputting on powerlock or lugs are somewhat common and it is not unknown for a neutral to fail.The fact that historically a LOT of those loads were rather harmonically right and you could be routinely running the three phase neutral at anything up to 200% of the most heavily loaded phase does NOT help (Yep, this can happen, consider that the third harmonic sums instead of cancelling in the neutral).Note that lost neutral upstream of an earth/neutral bond has ALL kinds of nasty implications around safety and the measures taken to control touch voltage, not just blown out kit, this is why exporting a neutral from an equipotential zone without taking precautions in TN-C-S/PME/MEN (Names vary) gets the stink eye for EV chargers.