Original subtitles

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