electrical testing

martz · Posted: Sat Sep 12, 2009 6:25 pm

when wiring up a new electric oven which is on its own MCB (which was isolated), i found that when i cut into the cable at the length i wanted, it tripped out the RCD. when continuity tested with test meter, i found continuity between earth and neutral. Is this what you would expect to find and can anybody explain why i am getting continuity between earth and neutral?

Thanks

flameport · Posted: Sat Sep 12, 2009 6:30 pm

Even with the MCB off, earth and neutral are still connected. Your RCD is designed to operate when there is a N-E fault within your installation, such as you cutting through a cable.

In most installations, E & N are connected together in one or more places - the supplier cutout, in the road outside, or at the substation supplying your property.

Guitarguy · Posted: Sat Sep 12, 2009 6:34 pm

if the house is on a PME supply, the neutral and earth is one conductor in the supply cable and both the neutral and earth tails go into the neutral side of the electricity companies cutout. That would explain continuity

cantab · Posted: Sat Sep 12, 2009 6:47 pm

EDIT: What I wrote in this post is WRONG, so ignore it. Others have explained why.

If the RCD tripped when you cut the cable then I reckon that cable was LIVE, and you're lucky to be (a)live yourself.

I'm no spark, but I think RCDs trip if there's a current imbalance between live and neutral. A dead circuit should have zero current in both.

londonboy · Posted: Sat Sep 12, 2009 6:49 pm

If it was live he woulda known about it!! N to E caused fault.

cantab · Posted: Sat Sep 12, 2009 6:55 pm

Can you explain? Sorry, I'm a bit confused here. Am I wrong in how an RCD works?

If a circuit's dead, there should be no current flowing in either live or neutral, right. Thus, shorting neutral to earth, still no current. Am I missing something?

(EDIT: You're right that if one cuts through a live cable that should be very obvious. I'm just trying to see how cutting a dead cable would cause anything to trip.)

ban-all-sheds · Posted: Sat Sep 12, 2009 7:00 pm

cantab wrote:

If the RCD tripped when you cut the cable then I reckon that cable was LIVE,

Quote:

and you're lucky to be (a)live yourself.

Quote:

I'm no spark

ban-all-sheds · Posted: Sat Sep 12, 2009 7:02 pm

cantab wrote:

If a circuit's dead, there should be no current flowing in either live or neutral, right. Thus, shorting neutral to earth, still no current. Am I missing something?

Yes - the fact that all the neutrals (on the RCD side) are connected together at the CU, so shorting one of them to earth shorts all of them....

BS3036 · Posted: Sat Sep 12, 2009 7:05 pm

As flameport mentioned, the MCB doen't switch off the neutral. The neutral will have a small voltage on it caused by current flowing through the (small) resistance of some part of the neutral cable. So if you join neutral to earth through a very small resistance it is easy for between 15 and 30 mA or more to flow from neutral (and not in the live), ie enough to trip the RCD.

Oh well.

cantab · Posted: Sat Sep 12, 2009 7:06 pm

Ah, I see. Thanks.

Johnmelad502 · Posted: Sat Sep 12, 2009 7:58 pm

ban-all-sheds wrote:

cantab wrote:

If the RCD tripped when you cut the cable then I reckon that cable was LIVE,

Quote:

and you're lucky to be (a)live yourself.

Quote:

I'm no spark

Wow, that was sooooooooo helpful.

davelx · Posted: Sun Sep 13, 2009 9:11 am

What BAS is saying is that unless you KNOW what you are talking about then better to keep quiet and learn from those who do.

ban-all-sheds · Posted: Sun Sep 13, 2009 3:06 pm

Not necessarily - sometimes doing or saying something that makes you look a right plonker can be a spur to learning things so as not to look a plonker again..

cantab · Posted: Sun Sep 13, 2009 3:46 pm

Indeed

ban-all-sheds · Posted: Sun Sep 13, 2009 4:07 pm

My words were a bit ill-chosen - I didn't think that your mistaken post made you look a "right plonker" - that's definitely too excessive - sorry about that....