⚡ TN Systems Explained: How Your Electrical Supply Stays Safely Connected to Earth
- MTS DNC ENERGY CONSULTANTS LIMITED
- Jan 3
- 3 min read
When it comes to electrical safety, one of the most important systems used in homes and buildings is the TN system. It’s the backbone of how electricity is safely distributed — and how faults are handled to protect people and property.
Clause 411.4 of I.S. 10101 outlines everything you need to know about TN systems. Let’s break it down in simple terms.
Integrity of Earthing in TN Systems
In a TN system, the safety of the entire installation depends on a reliable connection to earth. This connection is made through conductors called PE (Protective Earth) or PEN (Protective Earth and Neutral).
If the earth connection fails, the whole system becomes unsafe — so it must be:
Connected at multiple points
Installed to minimize the risk of breaks
Maintained by the electricity provider (DSO) if it comes from the public supply
Example:
The PEN conductor from the street is earthed at several locations to ensure continuity. If one point fails, others still keep the system safe.
Earthing the Neutral Point
The neutral point of the supply system must be earthed. If there’s no neutral point available, one of the line conductors must be earthed instead.
Inside the building:
All exposed metal parts must be connected to the main earthing terminal
That terminal must link back to the earthed point of the supply
Tip:
In large buildings, extra earth connections may not be practical — but equipotential bonding between metal parts can achieve the same safety effect.
Using a PEN Conductor
In fixed installations, one conductor can serve as both protective earth and neutral — called a PEN conductor.
But there are strict rules:
It must meet specific requirements (see Clause 543.4)
You cannot install switches or isolators in a PEN conductor
Why?
Because interrupting the PEN could disconnect both the earth and the neutral — creating a serious hazard.
Fault Loop Impedance Requirements
This clause introduces a key safety formula:
This formula ensures that:
If a fault occurs, enough current flows to trip the breaker or RCD
The disconnection happens within the required time (e.g., 0.4 s or 5 s)
Example:
If the fault loop impedance is too high, the breaker won’t trip fast enough — and the system becomes unsafe.
Acceptable Protective Devices
In TN systems, you can use:
Overcurrent protective devices (like MCBs or fuses)
Residual Current Devices (RCDs) — but not in TN-C systems
Important Notes:
If you use an RCD, the circuit must also have overcurrent protection (see Clause 430)
In TN-C-S systems, the PEN conductor must be split before the RCD — not after
RCDs must not be used on the load side of a PEN conductor
Tables to Know:
Table 41.2: Max fault-loop impedance for 230 V circuits ≤35 A (0.4 s disconnection)
Table 41.3: Max fault-loop impedance for circuits >35 A (5 s disconnection)
🧠 Why TN System Rules Matter
TN systems are the most common type used in homes and commercial buildings. These rules ensure:
The earth connection is always reliable
Faults are detected and disconnected quickly
Protective devices work as intended
People stay safe even if something goes wrong
📘 Summary (from I.S. 10101)
Clause 411.4 of I.S. 10101 covers TN system requirements:
The PEN/PE conductor must be reliably earthed
The neutral point of the supply must be earthed
PEN conductors must not be switched or isolated
Fault loop impedance must allow fast disconnection
RCDs and overcurrent devices are allowed — but RCDs must not be used in TN-C systems
These rules form the foundation of safe electrical installations across Ireland.
📍 Disclaimer
The content shared in these posts is intended for informational purposes only and should not be interpreted as design advice, specifications, or a calculation template. For professional guidance or design services, please contact us through our contact form.
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