Electrical Isolation Requirement and Neutral Wire

The controversial topic of isolation requirement and all those questions that comes with it:

• Is it sufficient to isolate the active conductors only in Alternating Current (AC) system?
• Does neutral require to be isolated in AC single or three phase systems?
• Do you have to isolate both negative and positive in DC system?

Regardless of in which country we practice, we as electrical engineers must take this matter seriously. The answer is not a simple Yes or No. Assessment must be done case by case. I will take you through number of National and international standards to further clarify the matter. Hopefully this will shed some light on isolation implication in our industry.

In this article I will mainly focus on role of Neutral conductor for isolation purposes. In general the intent of "neutral" conductor is to provide a return path for the unbalanced portion of the circuit current and also in some cases a protective earthling system.

AC System

To isolate a device/plant it is stated in Australian Standard (AS3000) and IEC 60364, all active conductors of an AC shall be capable of being isolated.

Now the question is this: What is an Active conductor?

The active conductor is defined as “Any conductor that is maintained at a difference of potential from the neutral or earthed conductor. In a system that does not include a neutral or earthed conductor, all conductors shall be considered to be active conductors”. This is quite clear but what about Neutral? We all seen neutral become active (figuratively speaking).

Now there are two scenarios, one is you do not switch the neutral during isolation and the other when you may. Note when it comes to switching it is left to you decide as per your system configuration.

A) Not switching the Neutral

1. If the conductor supplies the main (distribution transformer or similar)
2. If the neutral act as protective earth and neutral (PEN), some of you may know it as TN-C. This also applies to TN-C-S system where part of the system uses a combined PEN conductor, which at some point will split up into separate PE and N lines. This system also known as Protective Multiple Earth (PME) in UK and Multiple Earthed Neutral (MEN) in AU, see figure below.

One of the reasons we use PME/MEN system is to reduce the risk of electric shock in the event of a broken PEN conductor. Looking at the left schematic, if your isolation device (e.g. CB) is on the red line you can argue the protective earth is not broken during isolation although you isolated the neutral.

So as long as that neutral is grounded at one point and it is used as a protective earthling at the load end, we are not permitted to switch it.

B) Switching the Neutral

If you have not satisfied the conditions above in section “A”, you may now operate your Neutral considering below factors apply:

1. If the neutral conductor is connected to a multipole switch/breaker along with other active conductors. That switch shall have appropriate short circuit breaking and making capacity. All poles are linked together in the switch where all active and the neutral conductor are operated at the same time.
2. If there are multiple switches with one neutral switch or the neutral switch is physically separate to its corresponding active switch. You may operate that neutral switch as long as neutral contact cannot remain open when the active contacts are closed.

You may operate the neutral pole if the neutral is operated at the same time with other corresponding active conductors or if the neutral pole does not open before and does not close after the active pole. You can now start to see the reasoning behind operating and not operating the Neutral.

The Australian standard states “Where an item of switchgear is required to disconnect all live conductors of a circuit, it shall be of a type such that the neutral conductor cannot be disconnected or reconnected without the respective active conductors also being disconnected or reconnected”

Floating neutral

The neutral should only be connected to ground at one point such as MEN. This will create a path for current to return to service via ground. In a 3 phase system the grounded neutral will stabilize all three phases. In case of ground breakage voltage potential will be present. Similarly if during isolation we only break the neutral, we will have floating neutral. During this process we are disturbing the current path which would return from each phase via neutral. In this scenario usually the current will return via the other two phases which results in voltage imbalance. This is when you will read unexpected high voltage (~100v in a three phase 415V system) across your Earth and Neutral, which is quite dangerous and unsafe.

There are many reason for floating neutral the most common one, I would say is shared neutral wiring. Where electrician try to save time and money. In buildings these days most electrical equipment are non-linear with certain level of harmonic (digital world!). Not having dedicated neutral wire will means all these current will add up and return on single line. A line that is not adequately rated and will lead to overheating and high neutral to ground voltage and ultimately limiting your supply voltage.

See below example to further clarify the matter. Let’s say a technician would like to perform maintenance on Load A, i.e. single phase supply with Neutral wire and local earth.

The technician will isolate plant A by opening Switch A. It is possible for the technician to be electrocuted via Neutral conductor. That can occur due to existing potential difference or a fault at load B which creates a potential difference between distribution board, Load A or Load B. There are many other scenarios where there is system vulnerability. These situations usually will happen in places with old wiring and bad practices where the electrician try to save time and money!

For the same reason no fuse shall be inserted in a neutral conductor. So in case of fault you won’t lose your neutral and maintain continuity. Protective devices that incorporate a switching function in the neutral conductor shall comply with the requirements of section B.

What about a transfer switch?

Neutral switching would be 'required' (by codes such as NEC and AS), if the two sources are independently grounded i.e. separately derived systems. This is to avoid multiple neutral grounding and proper functioning of ground fault current sensing/protection.

What about the AC switch discounter for a PV inverter?

As per BS 60947 and AS, to isolate we must switch all live and neutral conductors.

DC system

Generally speaking both the negative and positive in DC circuit shall be capable of being isolated. In the following scenarios you may only operate one pole for isolation:

• Extra low voltage DC system
• Where one of the poles is connected to earth. E.g. some UPS system, earthed D.C. negative in battery banks and midpoint of PV string.

Note: earthing one pole will increase possible earth fault paths, and possible problems with commonly available European inverter types and internal earth fault detection circuitry.