Why Don't We Break Neutral in AC Circuits?

Neutral is connected to earth at some point, thus it has some value as a return path in the event of say and equipment earth being faulty. It's a bit like asking 'why don't we break the Earth connection' ?

Medical equipment (CE marked equipment, not 'permanently installed' ...used to be BS5724...can't remember the new spec' number) must have Live and Neutral switched and fused.
Arguably this protects against L and N being reversed. My personal view was (when I worked in medical electronics) that it was stupid and dangerous, at it was possible for the neutral fuse to blow, giving the appearence of 'no power' when in fact the equipment was still live.

How ever you try and protect there is always someone who will manage to get a shock.

I'm sure someone can postulate an opposite scenario/view point...

Del

Here in Spain, after the meter, every installation has a 2P or 4P main breaker! If it has a neutral, then that is classed as a conductor and is treated like a conductor! Old houses are the bane of my life at the moment, the ones that people have modified using any colour cable they can find! If I can't turn the whole thing off before I start sticking my hands in rustic junction boxes, then it's a good bet that things are looking dodgy!

I've done several systems for the HPA-RP (formerly NRPB) - they seem to spend some time debating internally whether single (L only) or double (L+N) fusing should be used. It's got to the stage where I draw something up (finger-in-the-wind), and wait for them to shoot me down at the design review. Can't get my head round it. I always switch L+N at the isolator (never any argument there).

Generally, I gather that they want double fusing except where the system includes a mains outlet for accessories (switched & controlled via the MCR). Apparently it's all hidden away somewhere in the regs (EN-XXX-Blah-Blah-2020, FKA BS 12345678)

Maybe I'm getting too old ...

I always switch L+N at the isolator .

Sorry old chap I never go back as far as the isolator...I get my butler to do that.

In our domestic set up ....
Each cct has the breaker on just Live. (including the cooker)
The electric shower* has L & N on RCD breaker
The incomming is broken L & N (at risk of stating obvious).

(Electric shower? What about the solar?...Mrs Cat has her own little shower room with a mirror and girlie lights all round it...I have to scrub in a tin tub out in the yard)
Del

In most equipment I have seen supplied with 3 phase power no neutral has been run. Only the 3 phase and ground. Unless they are using it for control circuit power no neutral is run. Most use control transformer. In the 3 phase application the power is from phase to phase not to neutral. In a single phase application if neutral was switch all the devices would be hot and have power to them with relation to ground. In failures where insulation is broken down the leakage current to ground may fry the ground wire. By code in the US it is allowed to be one size smaller than the power conductors.

"By code in the US it is allowed to be one size smaller than the power conductors"

Sorry, but can you please clarify - are you saying that the gound wire can be smaller that the L or N conductors?

One finds the same feature in some twin-and-earth cables in the UK: the earth conductor can be of smaller cross-section than the power conductors. With TT distribution systems that are fitted with an effective overall 100mA RCCD on the inlet, it's absolutely fine and complies with present Regulations.

L & N are sized for continuous use, whereas an earth connection will conduct only until the fuse blows, or the circuit breaker is tripped - not enough time for much heating.

Not too long ago I assisted a Power Engineer to solve a conundrum at the service entrance to a big lab loaded with computers, routers and such. The common thing was, that practically all that equipment used switching power supplies. First, we did redistribute the load among the 3 phases to get the balance under 5%. Then he remeasured the current on the neutral with a clamp-on meter. It still was higher than the current on any of the phases.!!! We could not figure it out, until using an oscilloscope and the current transformer we traced the voltage and current on all 4 wires. The switching power supplies distorted the 60 Hz badly and synchronously, causing the high harmonic current on the neutral, that was not permissible. The solution was a 3 phase harmonic transformer: low inductance has minimal effect at 60Hz, sharply increasing at higher harmonics. The current on the neutral dropped below15%, good enough for the electric code requirements. But when you squeeze something, something else got to give: It was observed, that the power supplies causing the problem started to switch at different phase angle, smearing the voltage waveshape.

There are cases, like delta wired motor, where a neutral has no place, but otherwise the neutral carries the balance of the 3 phases, which should be low, but can be anything. I would not switch it more, than a protective ground. And never mind, what a code said. My and my people's safety comes first. Trusted neutral and safety ground is a double insurance in my eyes.

Leveles, I think you meant to say the neutral carries the unbalanced phase current

when there many switching power supplies on the phases we use a super neutral,

that is we go one size larger to help take care of the additional heating perry

Yes, you can do that here to, as long as it does not leave the premises. I think, that is still a bad form.

In our case, we were inspected before, during and after the installation of the harmonic transformer to make it sure, the Electric Code was met, including the maximum permitted harmonics injected into the power lines by us.. If you met inspectors, you know there is little room and even less sense to argue with them.

Is guest in the UK ?

I was reading an old article on this topic yesterday which stated that breaking the neutral was de rigueur (?) in Europe but not in the UK and that Uk installers would be wise to move over to this standard on the basis that Europe would be insisting on it eventually. I've just checked the date August 1999 in Electrical Engineering. So eventually is presumably still on hold but there is a new Edition due out (18th I think) of the Wiring Regs.

Not everything Europe does with regard to electricity is good, just look at the antiquated plugs and sockets for one, the fact that many older houses have only 2 wire systems and not 3 for another here in Germany for example!!!!

I feel sometimes that Germany is in the dark ages after living in the UK.....and they wanted the UK to switch to the Continental style when we joined the EU!!!! I laugh my socks off!!!

I am not active enough in this area to be personally certain that breaking the neutral is good or not,(personal feel is towards "not") but I recently had my house rewired and both neutral and earth are NOT broken at any point in the new system installed!!!!

Except that most on/off switches on equipment are double pole......because you never know which is the live on many continental systems due to the shitty plugs and sockets!!

Even in some countries like France with the continental plug with the third pin, I never actually noticed if live was always on the same pin or not. In Germany, it can come on either.....

But I did not get the impression that this was what you were talking about with regard to breaking the neutral or was it?

Point taken. In Spain our rented apartment was wired 30yrs ago with various tack ons and there are a variety of sockets for which the plugs are virtually un-obtainable and I have had to make adaptors with what I could find.

In the article referred to I quote

The 3 main requirements of an RCD are that it should provide additional protection against direct and indirect contact with live parts and PROTECTION AGAINST FIRE in the event of a FAULT TO Earth.

This last gives the main advantage of the switched neutral. If a N to E fault occurs here will be a trip and cct will be safe until fault is remedied. Where only live is switched the RCBO will trip but the fault shall still be in cct within the rest of the installation.and current could still flow. Also the selectivity of upcct RCCb's will be lost as the main RCCB will trip as it will see the N - E fault as not cleared.

You wrote:-

This last gives the main advantage of the switched neutral. If a N to E fault occurs here will be a trip and cct will be safe until fault is remedied.

I do not understand your thinking here. I have ELCBs installed and as this is an old house, some of the wiring is/was only 2 wire with a link at each socket for earth to neutral.

My new ELCBs did not like that at all, I had to get some wiring replaced to stop them tripping and neutral is NOT switched here at all.....so what you wrote is simply not needed, it does not give you any more safety than NOT having it!!!

In fact, having a switched neutral COULD make things more dangerous as somebody else also pointed out, your Vacuum cleaner for example does not work, but could have the live phase still connected!! Sounds downright dangerous, at least in a house.....

Hi,

I agree with Del breaking the Neutral is very dangerous but becuase what you then have is an open live line and if you come in contact with something that completes the circuit there is a very good possiblity of shock. If you would like to read a good article about this go to http://wiki.answers.com/Q/What_is_neutral_in_electricity

There are also some safety control systems that after breaking the lines will contact the load side back to neutral (or ground) to guarantee the equipment to be worked on is at zero potential.

Imagine if there was a fault on the line before your house and the neutral became energized! This type of fault will not throw a domestic line until quite a bit of juice is flowing. This has happened on a line near here! Workers connecting a new line up a post to an existing line mistakenly connected the new neutral to a phase line! Now this neutral is connected to ground where there is a consumers main fuse box, The whole line starts to humm! It was discovered after the farmer contacted the contractor to find out why his new line was humming! Lucky for him he had the 3 phases and the neutral on a 4 pole main switch otherwise the poor electrician who was installing the electrics on the inside of the farm could have got a nasty belt.

Thank you for your elaboration. BUT, please forgive me for my reluctance to break the Neutral to Ground continuity, as a double insurance of grounding.

Thanks

Leveles,

I understand how things can be confusing, particularly in the circumstance that Andy of Germany mentioned. I fully agree with your reluctance to break the continuity between Neutral and Ground. BUT, I would add that this reluctance assumes an installation which is compliant with current code requirements.

As I have mentioned in other threads, the electrical codes are a constantly-evolving work. In the USA they are written by an organization which is not sponsored or funded by any governmental agency, the National Fire Protection Association. Membership includes people representing insurance companies, manufacturers, and governments; as well as engineers, citizens, trades people, etc. They follow a code revision cycle with comments and proposals being received, committees reviewing these and making recommendations, and member meetings where the proposals are voted upon. The code text is written with recommended or suggested wording so it can be adopted by a governmental body as the law in that jurisdiction. One confusing thing is that the frequency of adopting the newest revision varies from one jurisdiction to its neighbors, so some could always be on the newest while the adjoining city or county or state could be on a version that is 10+ years old. Electricians have to keep up with which versions are in use and where, because some wiring methods are permitted in one and forbidden in another...

I mention the above history, because it was common at one time for every lamp, fan, appliance, etc. to have only a 2-wire attachment cord, and for this cord to be able to be plugged into the wall with either wire on the hot and the other on the neutral. Various things were all done to eliminate the hazards from this---polarizing the plugs so they could only be inserted in one orientation, adding a safety ground to bond the metal parts of the appliance to the equipment ground, requiring the current-carrying neutral to be electrically separate from the non-current-carrying safety ground (the final exceptions to this were eliminated only in the last two code cycles), etc.

Another set of changes also was being done by all the manufacturers---to standardize the configurations of receptacles so there would be no confusion regarding the voltage, number of conductors, and current which that particular receptacle was intended to supply. Some configurations with multiple uses were designated as obsolete. This has led to some confusion because there are over 50 specific plug configurations available. But, when you know how they are organized there is no confusion at all (straight blade vs. locking, voltage, number of poles, presence of neutral, ampacity). Each manufacturer can have his own catalog numbering system, but all of them are made to the same standard with interchangeability when they have the same standard's configuration. I can only imagine the problems that can arise when you have adjoining countries which have historically-derived standards for these which are all different.

I mention the above, because I suspect your installation is somewhere near the earlier portion of this path we have been following. Breaking the neutral might break the ground also. But if it is done by a switch which has simultaneously broken all the other conductors in the circuit I fail to see the hazard. That is like unplugging the device. Just remember the main points--the (safety) ground is not normally part of the current-carrying circuit; the ground and neutral are bonded together only at the place where the power comes into the building and nowhere else; and everything which can become energized and touched by people is bonded to the ground. Do this, and switching the neutral simultaneously with all other conductors should be safe in all cases.

Regards--JMM

You wrote:-

the ground and neutral are bonded together only at the place where the power comes into the building and nowhere else;

I disagree, my infos point to the link normally being in the supplying substation. If your house has its own substation (possible) then you would be right, but otherwise, you are wrong as far as I am aware....

Having a link in every house would make problems with ground (and neutral!) loops and the like I would imagine....lets see what the others have to say!!

The link used in older houses in Germany, links a wire that goes to the frame, with a wire that is also neutral (I am pretty sure that In have it the right way round,) but only in the socket, nowhere else and as it is only connected with two wires to the switchboard, there is no link anywhere between (real) earth and neutral......

Andy,

The whole thing gets very complex. My copy of the IEE regs is now well out of date (16^th edn, 1992! - must replace it soon), but under Wiring Systems in the Definitions section it describes five different systems - TN-C, TN-S, TN-C-S, TT and IT - each of which has a different arrangement for earthing.

When I sort out my update, I'll try to send you a copy of the relevant bits. Maybe it will be "harmonized" by then!

Regards, John.

[PS

Q: What's the fastest liquid on Earth? A: Milk - because it's Pasteurized (say past-your-eyes(d)) before you see it!

Sorry ]

So a blonde hears that milk baths are good for the complexion and places an order with her milkman for 25 gal. The milkman sees the order and asks the blonde, "Are you sure you want 25 gal., not 2.5 gal?"

"Yes, I want 25 gal.," the blonde replied.

"What are you going to do with 25 gal. of milk?"

"I'm going to take a milk bath."

"Oh. Do you want it pasteurized?"

"No, just up to my tits is fine. I can splash my face."

Andy,

That is part of the fun and education in an international group such as CR4. In the USA, the National Electrical Code requires every service neutral to be bonded to ground at the service equipment if it can be done so that the conductor with the highest voltage to ground is not over 300 volts. There are a bunch of rules beyond this and exceptions also, but you will almost always see a grounded neutral at the service disconnect.

That way, a fault or short circuit to ground will return via the shortest ground path back to the service equipment and trip the CB or "blow" the fuse to clear the fault. If the bonding between neutral and ground is further away, the ground path resistance would be higher, so the clearing of a fault would be more difficult. This also means that the voltage of neutral to ground is very low and least likely to cause any shock hazard. (Except the known problem of "tingle current" in milk barns.)

Your point is well-made that this bonding poses a potential for ground currents and ground loops on the supply side. Multiple grounding gets discussed, but I know of no recent attempts to change this method of grounding the neutral because of problems with multiple grounds.

Of course, where instrumentation or low-voltage signals are involved and connect between buildings, there is a need to keep from having ground loops on these lines. Often signal isolators are used, or fiber-optic links are specified, to avoid these types of problems.

I appreciate your input--JMM

BECAUSE WHEN EVER THERE IS A NEUTRAL BREAK,, THE NEUTRAL ACTS AS PHASE.

depending on the type of earthing the neutral always becomes live after the point of breakage. and the obvious is it always complete the circuit.

YOU JUST DONT BREAK THE NEUTRAL!! IF SO THE VOLTS WILL RISE BEYOND THE STATUTORY LIMITS CAUSING DAMAGE TO ELECTRICAL EQUIPMENT!