Introduction
In AS/NZS3000, it’s specified that switching the neutral conductor of consumer mains is not allowed. This applies similarly where the neutral conductor is used as a combined protective earthing and neutral (PEN) conductor for protective earthing of any portion of an electrical installation.
"⚡⚠️ Important lesson from AS/NZS 3000: Never switch the neutral conductor in consumer mains! From incomplete earth loops to energized earthing systems, it's a recipe for trouble. Discover the 'why' behind the rule. 📚 #AS/NZS3000 #SparkyCalc #SafetyMatters"
What the standards say
To understand why switching the neutral is a very bad idea, you need to understand the earth fault loop system and the MEN system. For a deeper understanding of the MEN system, please refer to the MEN and earth fault loop impedance blog posts where we take a deep dive into these topics.
Referring to the simplified MEN system shownd in the figure below, the earth loop path consists of parts AB, BC, CD, DE, CE, FG, and GH. The consumer mains’ neutral conductor is an important part of this earth loop.
If a fault occurs in the system and upstream protection needs to trip, current must pass through the consumer mains’ neutral to reach the neutral star point in the transformer. If, for example, the consumer mains is interrupted by a switch at the indicated point in the diagram below, the earth loop becomes incomplete.
If a fault occurs at any point in the circuit, for example at the point marked D, as a result, the installation’s earthing system becomes energized when current tries to flow through the ground to the transformer’s star point. As this current increases, so does the voltage across the earthing system, due to the impedance of the ground.
The voltage across the earthing system can reach the full supply voltage. It’s unlikely that enough current will flow through the ground to activate upstream protection. This situation can lead to energized earthing systems, posing a risk of electric shock.
As a result, a potentially lethal current might flow to the ground for an extended time without any protection equipment activation. I think you now get the idea why switching the neutral consumer mains is not recommended.
Exceptions to the rule
Like everything else, there are exceptions to certain rules. AS/NZS 3000 provides specific situations where it is perfectly fine to switch the neutral, such as in cases involving backup generators or solar systems.
Below, we have some typical drawings from AS/NZS 3000 where the neutral is being switched.
Rounding Up
That’s all we have for you today on this topic. If you’ve learnt something new from this post, you might also be interested in other articles we offer on related subjects available on our blog’s homepage.
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