Correct Isolation Procedure

Essential steps for safe electrical isolation

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1. Pre-Isolation Preparation

2. Shut Down and Isolate PV Systems

3. Isolate BESS

4. Disable or Isolate EV Charge Points

5. Shed Loads on Other Distribution Boards

Step 6: Approach the Intake Position

Step 7: DNC and NET Testing Before Work Begins

Step 8: Prove Dead - If no DNC detected

Step 9: Reporting and Next Steps

Overview

Modern electrical work requires enhanced isolation verification that accounts for the possibility of diverted neutral current.

This procedure ensures that before any work begins on an electrical installation, all sources of energy have been identified, controlled, and verified as safe — including potential diverted neutral currents that may energise bonded metalwork or earthing systems even when the installation appears isolated.

The steps outlined below apply to both domestic and commercial installations, with particular attention to installations containing:

⚡ Solar PV system
⚡ Battery Energy Storage Systems (BESS)
⚡ EV charge points (including V2L/V2G capability)
⚡ UPS units or standby generators
⚡ Multiple distribution boards with shared neutrals

⚠️ Important: At this stage, do NOT assume the installation is safe. Proceed to DNC testing before any further work.

Step 1: Pre-Isolation Preparation

1.1 Confirm additional energy sources

Before approaching the intake position, identify whether the installation includes:

⚡ Solar PV systems
⚡ Battery Energy Storage Systems (BESS)
⚡ EV charge points (including V2L/V2G capability)
⚡ UPS units or standby generators
⚡ Multiple distribution boards with shared neutrals

⚠️ Important: These systems can back feed, auto reconnect, or energise circuits independently of the grid.

Step 2: Shut Down and Isolate PV Systems

PV arrays generate energy whenever illuminated. They must be fully isolated before any other work can take place.

2.1 Follow inverter shutdown procedures

2.2 Consider hybrid inverters

Hybrid systems may continue energising circuits from the battery even when the grid is down. Ensure both AC and DC sides are isolated.

Step 3: Isolate BESS

BESS units can supply significant fault current and may automatically reconnect.

3.1 Perform full BESS isolation

3.2 Verify isolation

Some BESS units have internal capacitors or delayed shutdown sequences. Wait for confirmation indicators to extinguish.

Step 4: Disable or Isolate EV Charge Points

🚗 EV chargers introduce several unique risks:

⚠Internal relays may close unexpectedly
⚠PEN fault detection devices can introduce voltages during testing
⚠V2L/V2G systems can energise circuits independently

4.1 Isolation steps

Step 5: Shed Loads on Other Distribution Boards

In multi DB installations, parallel neutral paths can remain energised even after the main switch is off.

5.1 Isolate all secondary DBs

Shed the loads on each of these boards
Switch off main switches on all sub boards
Disable UPS units, standby supplies, and load shedding systems
Confirm no automatic transfer systems can re energise circuits
Apply isolation lock off to all main switches

5.2 Be aware of bonding

Bonded metalwork can still carry diverted neutral current even after all Distribution Boards are isolated.

Step 6: Approach the Intake Position

Once the PV, BESS, EV, UPS, and all Distribution Boards are isolated, proceed to the intake.

6.1 Isolate the main supply

Step 7: DNC and NET Testing Before Work Begins

With the installation isolated, test for diverted neutral current and net currents.

7.1 Tools Required

🔧 Clamp meter (for bonding/earth currents)
🔧 Voltage tester (for touch voltage checks)
🔧 Power quality analyser (for harmonics and imbalance)
🔧 Insulation tester (for neutral integrity checks)

7.2 Step-by-Step Testing Procedure

1 Visual Inspection

Look for:

2 Measure Neutral and Earth Currents

3 Test Exposed Conductive Parts

Check:

💡 Voltage between bonded metalwork and true earth may indicate DNC/NET.

4 Load Balancing and Phase‑Neutral Checks

Using a power quality analyser:

Step 8: Prove Dead - If no DNC detected

Test between:

🚪 GATEWAY: You are either clear to continue your task or you will be putting the cover back on the distribution board and calling 105

Step 9: Reporting and Next Steps

If DNC, NET, or a suspected PEN fault is identified:

9.1 Report immediately

📞 Call 105 (UK DNO emergency number) and report:

9.2 The Client

Once the consumer unit or distribution board cover has been replaced and the installation has been left in a safe, isolated condition, it is important to recognise that you are now dealing with a supply‑side fault that is outside your control and outside the scope of your work. You cannot rectify the issue, and you cannot remove the underlying hazard; it is almost certainly affecting multiple properties in the immediate area.

At this stage, your responsibility is to ensure the client understands the situation clearly and safely. You should explain that:

A supply‑side issue has been identified and reported to the Distribution Network Operator (DNO)
Until the DNO attends and resolves the fault, no one should touch exposed metalwork, bonded pipework, or any conductive parts that could become energised
The installation must remain isolated, and the client should contact you immediately once the DNO has completed their work, so you can either remove your isolations or continue with the original task

This approach ensures the client is informed, protected, and aware of the next steps, while maintaining professional boundaries and compliance with your duty of care.

9.3 Provide supporting evidence

📋 Voltage readings
📋 Bonding conductor current measurements
📋 Photographs of damage or overheating

9.4 Do not re-energise the installation until deemed safe by the DNO to do so

9.5 First steps upon returning

When you return to the installation after the DNO has attended, your first priority is to verify that the installation remains safely isolated and that none of your previous safety measures have been disturbed. Proceed methodically:

1. Confirm All Isolations Are Still in Place

Before touching anything, visually and physically check every lock‑off, tag, and isolation point you previously applied. Nothing should be assumed — confirm each isolation individually.

2. Check for Residual or Returning Voltage

Use a non‑contact voltage tracer (light pen) on:

If the device indicates the presence of voltage, treat the installation as potentially energised and reassess before proceeding.

3. Measure Current on the Main Earthing Conductor

Use a clamp meter to check whether any current is still flowing on the main earth conductor.

4. Verify Integrity of Bonding Connections

Confirm that all bonding conductors remain securely connected to:

Any loose or disturbed bonding must be corrected before proceeding.

5. Assess Whether Conditions Have Changed

Compare your new readings with the measurements taken before the DNO attended. If:

Any loose or disturbed bonding must be corrected before proceeding.