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With breaker off, is it safe to measure resistance from Neutral to Ground

With breaker off, is it safe to measure resistance from Neutral to Ground

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  #1  
Old 12-23-20, 07:42 PM
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With breaker off, is it safe to measure resistance from Neutral to Ground

I installed a new outlet in my garage today. I cut the power to the circuit at the main panel and then verified I had no voltage between hot and neutral, hot and ground and neutral and ground. I wired up the new outlet, restored power and everything is as expected. The whole time I was wondering what I could measure to make sure everything was wired correctly.

When the breaker was off, would it have been safe to measure the resistance between the neutral and ground with a multimeter to ensure continuity? My gut says yes, but I'd like to confirm. Is it never a good idea to do this when the circuit is live?
 
  #2  
Old 12-23-20, 07:46 PM
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Yes it is safe.
Technically, it is also safe (for the meter) to measure on live circuit, but wouldn't recommend.
 
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Old 12-23-20, 08:17 PM
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Technically, it is also safe (for the meter) to measure on live circuit
It is safe for some meters to measure resistance on a live circuit but never assume it.
I have seen inexpensive meters explode.
 
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Old 12-24-20, 08:18 AM
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Just an FYI, when measuring the resistance between neutral and ground you will find it is VERY low. (close to zero) This is because they are connected together in the main panel.
 
  #5  
Old 12-24-20, 08:26 AM
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I like to run some current through to ground for a better test. Especially, when faced with an older EMT system with no ground wire. But this won’t work on a GFCI breakered circuit.
With only an Ohmeter, you need to be looking at just a few tenths of an Ohm.
 
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Old 12-24-20, 03:23 PM
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I assume you do this to see if the ground will properly ground in the event of a fault? Do you need a specific equipment to test this or can you just put a small DC load across the neutral and ground and measure the voltage drop?

Without a GFCI circuit breaker, is there any way to tell if the ground and neutral are shorted between the receptacle and the panel? I suppose measuring the presence of any current in the ground wire right before it's tied to the neutral (in the main panel) would tell me.
 
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Old 12-24-20, 04:17 PM
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Is there a problem you are trying to locate ?
Cable is rarely shorted.

If you want to check neutral vs ground. Connect a 100w light bulb from hot to neutral and hot to ground.
Is it the same brightness ? Yes.... you're ok.

If you have a clamp on current meter..... check the flow thru the light with it connected hot to neutral and hot to ground. If the current is the same.... you're ok.
 
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Old 12-25-20, 06:44 AM
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Connecting a 100 actual watt (i.e. incandescent) light bulb from hot to ground ... If the difference in brightness is noticeable compared with connecting it hot to neutral you know that there is a deficiency. But if the brightness is about the same it would be worth measuring (across the light fixture terminals or plug prongs) using a voltmeter.

Using a hair dryer on high heat would be a more rigorous test compared with using a light bulb.
 
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Old 12-25-20, 09:47 AM
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Using a hair dryer on high heat would be a more rigorous test compared with using a light bulb.
That would a 100% confirmation of ground continuity.
 
  #10  
Old 12-25-20, 08:38 PM
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Generally, yes.
If you have a "multi wire branch circuit" setup (generally in an older house), or incorrect wiring" there IS always a chance that the neutral can be at line voltage.



see
Multiwire Branch Circuit - Electrical 101
 
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Old 12-26-20, 09:10 AM
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Thanks all.

I now understand why pigtails and wirenuts are required for the neutral but just preferred for the hot.

In the Multiwire branch circuit diagram shown, if a neutral disconnected from either of the bottom terminals of the two left receptacles and someone plugged something into the far right one, would they all become 240V?
 
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Old 12-26-20, 09:19 AM
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Sort of. What would need to happen if the neutral is disconnected is there would need to be a load connected to both the black and red circuit. You would then have a 240-volt series circuit.
 
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Old 12-26-20, 09:29 AM
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No wiring defect can cause 240 v to earth.
But, in the larger view, there is a “open wire” defect that can cause a significant exposed 120v to earth condition. That is a neutral/ground open on a dryer or range 3 wire feed. That is why 4 wire feeds are now specified. Also, MWBC now require dual pole breakers in the pix above. If one side trips out, both sides go dead.
For DIY and even for electricians, a source of shock potential is working with branch circuit neutrals. If a neutral path is opened on a crossed/mixed neutral branch, that neutral wire can become live. Just some points to watch for.
 
  #14  
Old 12-26-20, 10:03 AM
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Be careful at subpanels. IF your subpanel has a good ground but a loose neutral, a significant amount of amperage will flow between neutral and ground when you bridge them.

Normally, the metal subpanel box IS bonded to the main ground BUT the sub panel neutral bar is NOT bonded to subpanel box. There can be a voltage difference between ground & neutral. All the subpanel circuit's neutral wires are connected to the neutral bar, so there CAN be a significant amperage involved.
So, at a subpanel, even though you turn off the circuit breaker and physically pull it off the hot rail, the neutral wire for THAT one circuit is still connected to the neutral bar and to ALL of the current flowing through the subpanel via the subpanel neutral bar.
IF the subpanel ground connection has a better connection than the neutral AND it's a hot summer day so you're sweaty AND you're touching that well grounded metal sub-panel AND you touch a disconnected return, YOU suddenly become a "better conductor" than the return bar and a portion of the current that WAS going through the bus bar will try a short cut to ground by flowing through you.
Don't ask me how I learned this...
 

Last edited by Hal_S; 12-26-20 at 11:00 AM.
  #15  
Old 12-26-20, 10:32 AM
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Dinko: Never use ohm/resistance meter to measure anything until first verifying there is no voltage
Instead, first at test point, use volt meter or short circuit with wire. If none then check resistance.
 
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Old 12-26-20, 10:53 AM
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Thanks. That's good advice. I always took it for granted that when I trip the breaker that both hot and neutral are open when in fact it's just the hot that is open.

Let's say I want to demolish some MC conduit. I start by killing the power to it. I then use a 120V reciprocating saw on a separate live outlet to cut the "unpowered" conduit, nails used to hold metal JBs, etc. If the neutral of my saw is in contact with its metal casing, which would not be easily detectable, could the current in the saw return along the neutral (or ground) of what I'm demoing? If I grab some MC with one sweaty hand while touching metal on my faulty tool with the other sweaty hand, am I gonna regret it?

 
  #17  
Old 12-26-20, 11:10 AM
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Originally Posted by dinko
If I grab some MC with one sweaty hand while touching metal on my faulty tool with the other sweaty hand, am I gonna regret it?
On a bad subpanel; possibly. The entire current flowing through a bad subpanel with a loose neutral will flow through you to get to ground, with a voltage proportional to the difference in ohms between ground & return.

That is why, on a subpanel, you flip the hot breaker AND unscrew the return wire from the bus bar.
 

Last edited by Hal_S; 12-26-20 at 11:29 AM.
  #18  
Old 12-27-20, 09:23 AM
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For safety, peace of mind and convenience all my sub panels are fed from individual breakers on main panel.

Reason for adding 240 volt sub panels was to get more breaker space in main panel.

Take 4 circuits to far end of home from main and put on local, nearby subpanel. Gained 2 breaker spaces on main. All sub panels are fed from individual breakers on main panel.

To work on a subpanel, just turn main panel breaker to it.
 

Last edited by doughess; 12-27-20 at 09:48 AM.
 

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