DB Board Breaking Neutral

[BobZA]

Hi All,

I keep hearing that we now supposed to break the neutral on ALL circuits in a DB … i.e. install double pole circuit breakers as a general rule.

I don't see any reference in SANS 10142 that confirms this … (apart from fixed appliances etc.)

Can anybody confirm this ? And if not specifically mentioned in SANS … what is the general standard for lighting and plug circuits in commercial buildings ?

Thanks

[GCE]

Hi
Plugs and lights circuits are generally 20 amp and 16 Amp on any site
Besides the normal fixed appliance and generator circuits there is a requirement under lighting.

Breaking of the neutrals can happen and be compulsory on 3 phase lighting circuits - I have heard the interpretation previously that all light circuits must be double pole but disagree and don't believe that the code is worded as such.Besides the fact that the note just seems to throw more confusion.
I read the regulation as to stop the possibility of having a floating neutral when working on a 3 phase lighting circuit.

6.14.1.2 Each identified group of single-phase luminaires supplied from a
multiphase supply that also feeds other luminaires, shall be controlled by a
local multiphase disconnecting device.
NOTE The disconnecting device should disconnect all live conductors that feed the
group of luminaires, including the neutral, in order that maintenance work can be
carried out without switching off all the lights

[Justloadit]

Besides SANS, in my book it is good practice to have double pole disconnect devices in the DB board for single phase circuits.
Effectively single phase DB boards are ultimately fed from a 3 phase supply, and with the prevalence of cable theft, where thieves are targeting only the removal of the Neutral because it is safe to do so, cause the Neutral in loads down the line to cause dangerous voltages being back fed from 3 phase loads, and simply disconnecting the Live, does not protect the single phase DB load unless the Neutral is also disconnected as well as part of the protection.

Had this problem last week, with the Red phase destroying many equipment that was connected on this phase due to a supply fault.
I was not able to get the actual issue from the municipality, but took them 2 days to fix the problem. Even had TV catching fire due to the fault, and a light switch catching fire which was supplying a number of fluorescent lights. Interestingly the earth leakages did not trip, only breakers due to the overloads once the equipment failed.

[BobZA]

Thanks Guys. I find SANS a difficult read.

Elsewhere it says "NOTE 3 Unless specified elsewhere in this part of SANS 10142 for a particular application, the protective device need not disconnect the neutral conductor". This is not very helpful as it is very difficult to find where it does specify to break the neutral leaving a lot to our own interpretation … which should not be the case.

then "6.9.2.2 In the case of a single-phase circuit, the disconnecting device shall disconnect live and neutral." Now what does this refer to ? If it refers to the DB main disconnecting device then fair enough. But it says circuit … so I may interpret a normal 16A lighting circuit as a "circuit" then then this "circuit" must have a double pole disconnecting device?

[ians]

My advice...be very careful disconnecting neutrals...especially on 3 phase systems... i have seen a few which didnt end well for the sparky.

Its like thinking you are too clever and dont switch off the light circuit... but instead just turn off the switch...thinking all is good... the light in the room next door is switched on and it bites you.

I have also seen a few nasty outcomes with sparkies working on single phase socket outlets in office blocks which are fed via a 3 phase system... they get all the neutrals mixed up and dont label the wires... it happened to me a long long time ago... I blew all the computers in an office next door to where i was working on the same floor.

[Dave A]

then "6.9.2.2 In the case of a single-phase circuit, the disconnecting device shall disconnect live and neutral." Now what does this refer to ? If it refers to the DB main disconnecting device then fair enough. But it says circuit … so I may interpret a normal 16A lighting circuit as a "circuit" then then this "circuit" must have a double pole disconnecting device?

I suggest the full reading of 6.9.2 gives us the context.

6.9.2 Disconnection of neutral conductors

6.9.2.1 A neutral conductor shall not have a single-pole disconnecting device.

6.9.2.2 In the case of a single-phase circuit, the disconnecting device shall disconnect live and neutral. In the case of a multiphase circuit, the disconnecting device shall disconnect all the phase conductors but need not disconnect the neutral conductor in an installation connected to a supply system in which the neutral conductor is earthed direct (see the TN system in annex J).

6.9.2.3 A disconnecting device used in a supply system in which there is no direct connection between earth and any live conductor shall disconnect all the live conductors (see the TT and IT systems in annex J).

NOTE In a safety supply, none of the conductors are connected to earth, so any disconnecting device in such a circuit has to disconnect all the conductors.

There is no instruction here that every circuit requires a disconnecting device. The purpose is to instruct which conductors any disconnecting device must disconnect given the situation in which they have been placed.

An observation on disconnecting the neutral in 3 phase circuits - the neutral should* be the last conductor to break and the first conductor to make. If this isn't written into SANS 10124-1 somewhere already, I suggest it should be.

*Note that "should" becomes "must" per the requirements of some power supply authorities for cross-over switches in installations with alternative supplies.

[Rifrug]

As per my understanding of SANS the disconnecting device is aimed at a isolator.
Easy examples is, if you fit a isolator for a single phase geyser circuit it must break live and neutral.
If you fit a isolator for a three phase air-con unit, it is allowed to break only phase conductors and does not need to break neutral. But you may break the neutral if you prefer.
Does this sound correct?