Open Neutral

here i'm using dc with resistive loads to show the effect of an open neutral my little setup here is laid out to mimic a three wire single phase ac system the type we use for homes or apartment buildings some of them and even some small commercial buildings if i have the same number of lights energized on each side a balanced load as we call it they stay the same whether the neutral is connected or not but if i have a different number of loads on each side an unbalanced load and disconnect the neutral one side gets brighter the other dimmer but also notice that only the neutral connected loads are affected the line-to-line loads don't change with the connected neutral open neutral connected open i'll disconnect some of these to show another aspect of this even with the neutral disconnected the open neutral if equal loads are energized from each hot wire with a shared neutral there doesn't appear to be a problem but when little johnny jr shuts off his light mom and dad's room goes dim and annie's gets real bright when annie's ready for bed and shuts off her light well mom and dad's lights go out as well even though their switches appear to be on let's reconnect these and break down what happened here with the loads balanced and the neutral open let's see what the current is i've got about 0.8 something amps here and the same here 0.8 something amps so these amps have to leave the source go through these two loads cross the neutral there's nowhere to go here so the amps have to keep going through these two loads and back to the source they're working like a big series circuit these two loads yes are parallel but this group of paralleled loads is in series with this group of parallel loads let's reconnect the neutral and see what the current does still have point eight something amps here going through these two comes to the neutral what does it do here point eight something amps here looks like it keeps going through here even with the connected neutral is there anything running here practically nothing this only brings back the unbalanced load so when you're balanced it still operates like that series circuit i was talking about if i disconnect one of these lights however here i only have 0.4 something amps half the load i turned one off here do i still have my point yep 0.8 something amps here so where did that other half of the load go 0.4 something amps because they're unbalanced it went on the neutral if i disconnect the neutral what happens to my current 0.5 something amps here and 0.5 something amps here it's equal it's that series circuit all the current has to go through this one light that's why it's burning so brightly it comes to the neutral and that current has to split up between these two they each get half the current and then go back to the source i also want to check some voltage here across this light we've got about 7.4 volts and across these lights less than two just under two volts both of them should be the same they're in parallel yeah just under two volts we're gonna have to go to the white board to explain that voltage difference but there's one other voltage measurement i want to take here i'm going to go from the neutral to the ground neutral should normally be the same potential but here i'm showing 2.7 volts now in a 120 240 volt system a couple of volts between ground and neutral is nothing but when we're dealing in these lower voltages that's a substantial amount so you can have some serious voltages where you don't think you'll have them when you have an open neutral scenario here i'm using numbers that we might expect from our three wire single phase 120 slash 240 volt ac system the 120 volt loads can run here and also here independent of each other if we keep a good neutral we can also use the full source voltage along the lines and run loads at 240 volts but these two 40 volt loads don't use the neutral so they don't play into this example i'll remove them for right now we want to make sure we recognize we have balanced loads here same voltage sync current and when we have balanced loads will any current flow on the neutral no the unbalanced current flows on the neutral this seven and a half amps would come from the source go down through this load through this other one here back and forward because it's ac but nothing would flow on the neutral so if i open the neutral would they continue to work just fine would it appear as though all is well and yes it would because we could go to this other side and consider the total source voltage of 240 volts and we have a series circuit so 240 volts getting split between two loads with equal resistances will mean that they each get the same voltage drop so they will get their rated voltage and operate properly and you could fill out the other values based on your series circuit rules but if the resistances were not the same let's say they were different with the open neutral and just operating as a simple series circuit would the voltage drops be the same no with different resistances you would have different voltage drops and that would throw everything out of whack the other thing i want to bring up here is that if i turn the switch off to turn off this one load it would turn off but what would happen to this load could it keep going no it's got an open circuit no current could flow to it so whether i turn off this switch or this switch both of the loads would turn off what i'm going to do here now is get rid of this load and replace it with what we had on the demonstration board with a load here and two loads up here that are the same as this one let's see how that works now i'm going to put these rated values down here so that we don't forget them here we have something similar to what we had on the demonstration board three equal loads two of them from l1 to neutral and one of them from l2 to neutral with a connected neutral no problem we'd have 120 volts here 120 here everything would be fine according to the ratings but how do we get these funky voltage drops well a quick bit of analysis the total resistance in the parallel portion here is 8 ohms in the parallel portion and that 8 ohms total here is in series with this 16 ohms to give me a total of 24 ohms once i get the current then we configure the voltage drops volts divided by ohms equals current 10 amps you can calculate power on your own if you want but the 10 amps leaves the source and goes here because these are equal resistances five amps will go down each side five vents here and five amps here will come back together and go through this load here all 10 amps go through here continue back to the source and then they keep going back and forward but what are the voltage drops huh ohms times amps gives me volts right 160 volts that's way over its rating that's going to cause a problem and how about up here way below its rating so what does that mean for power this one is operating nearly twice its rated power and these ones less than half the rated power no wonder those two lights were so dim and this one extra bright but there's one thing here that doesn't match with the demonstration here my voltage drop is twice what it is on the parallel portion in the demonstration it was more than three times almost four times as much why the discrepancy well when a resistive load gets more voltage and more current more power is it going to run hotter so the temperature is hotter than its rating and therefore its operating resistance should increase right with heat increase doesn't resistance increase and when these are operating below their rated power they'll operate below their rated temperature so their operating resistance would be lower and that would exacerbate this voltage drop problem this would actually be more than 160 and these would be less than 80 volts and that difference in voltage drops between the two halves is part of the reason that the neutral can become very dangerous there is now a voltage potential between the neutral and any ground along the system because they're connected together at the source we normally expect the neutral and ground to be the same potential or within at least a couple of volts but no this can create a danger so always watch out for that and the final thing is we could have some loads connected here that would be unaffected by the open neutral open neutral only effects loads downstream from where it's disconnected thank you hope that's helped to explain some of these funky voltages created by the open neutral

Channel: Dave Gordon

Views: 820,268

Keywords: What is an Open Neutral, Why is a Neutral important, Open Neut, Open Grounded Conductor, Open Neutral on 120/240 V System, Split-Phase Open Neutral, Center-Tapped Open Neutral

Id: LJvyb_WujZg

Length: 11min 41sec (701 seconds)

Published: Tue Feb 15 2022