Why DC Isolators Matter in UK Solar: Fire Safety & IMO Reliability

Why DC Isolators Matter in UK Solar - And Why IMO Leads in Safety

Safe isolation is at the heart of every compliant PV installation. While attention often focuses on the AC side, it is the DC side that presents the greater risk. 

Independent research, including BRE (2017) and the RC62 Joint Code of Practice (2023), shows that the majority of PV-related fires in the UK originate on the DC side of the system, most commonly due to faulty isolators or connectors.

By specifying the right DC isolation, installers can reduce risks, avoid unnecessary call-backs, and ensure long-term system safety.

What is a DC isolator?

A DC isolator is a switch-disconnector on the PV string side. It separates the array from the inverter for inspection, testing, maintenance, or emergency shutdown.

Unlike AC, DC current does not naturally pass through zero. Without proper engineering, opening a DC circuit under load can sustain an arc long enough to ignite a fire. For this reason, PV isolators must incorporate:

• Snap-action switching (<5 ms) for rapid current interruption
• Arc chutes to extinguish plasma safely
• Lockable OFF handles for safe servicing
• Weatherproof, UV-stable enclosures for outdoor environments

Compliance in the UK: Standards That Matter?

For UK solar professionals, two standards are essential:

• BS - BS 7671 (paid access) - pratical installer guidance on safe PV installation.
• BSI - BS EN 60947-3 (paid access) - standard for switch-disconnectors and isolators, core for PV duty.

Put simply: if an isolator is not marked IEC/BS EN 60947-3, it is not suitable for PV duty.

Why DC Isolation Demands Special Attention

DC isolation carries particular challenges: 

• No natural zero crossing → DC arcs don’t self-extinguish and need snap-action isolators to break safely.
• Parallel string back-feed → In parallel strings, faults can drive back-feed currents above isolator ratings, so installers must use DC-PV1 or DC-PV2 devices.
• Outdoor installation → UV and moisture cause cracking, corrosion, and arcing, leading to many DC isolator failures.

 
Best practice in the UK includes:

• Limiting external DC isolators unless specifically required (inverter-integrated units are often safer)
• Correct rating — always above Voc and Isc at lowest site temperature
• Careful installation — IP-rated glands, correct torque, and bottom cable entry for water protection

Quick Installer Checklist

• Voltage rating: always above string Voc at lowest site temperature.
• Current rating: Sufficient for Isc and back-feed currents.
• PV categories: DC-PV1 for single strings; DC-PV2 for parallel strings.
• Safe operation: lockable OFF, clear labelling, easy access for maintenance and emergencies.
• Correct installation: IP66 + enclosure, UV-resistant housing.
• Clear labelling (e.g. “PV array dc isolator”). See our PV Labels.

Where Fires Start — and How to Prevent Them

UK evidence shows that external DC isolators are the leading source of PV-related fires. Typical causes include:

• Moisture ingress into enclosures
• Cross-mating of incompatible connectors
• Loose or under-torqued terminals
• Poor crimps or incomplete connector engagement

Best Practice for Installers

In UK practice, installers should avoid putting a PV DC disconnector in any location that increases fire, water-ingress or access risk. 

• Avoid placing DC isolators beneath arrays unless required
• Do not mount in water-tracking locations (gutters, drip lines)
• Use bottom-entry cabling outdoors to minimise water ingress
• Only use matched connector brands and proper crimping tools
• Where possible, rely on inverter-integrated isolation

IMO DC Isolators UK: True DC Safety for Solar PV Systems

Among UK PV isolator brands, IMO is widely trusted by installers and wholesalers. IMO stands out because it combines purpose-built PV design, proven compliance, and real-world durability.

Poorly made or incorrectly installed isolators often fail under load or let in moisture, which leads to the majority of PV fire incidents. IMO has built its reputation by eliminating those weaknesses.

Key reasons installers choose IMO:

• “True DC” design & safety: patented snap-action (typically <3–5 ms) for effective arc suppression.
• Built to Last: UV-resistant materials and IP66/IP67 enclosures for harsh outdoor conditions.
• Proven trust: selected by leading inverter manufacturers and installers worldwide, with a track record of millions of safe installations worldwide.
• Certified compliance: certified to the highest UK and international standards (IEC 60947-3 and EN 60364-7-712).
• Range & flexibility: IMO offers a wide range of DC isolators for the UK residential and light commercial markets.
• Installer-friendly → lockable handles, spacious terminals, clear labelling.
• UK availability: widely stocked; special trade pricing available from Alternergy.

Choosing the Right IMO DC Isolator

Match the isolator rating to the expected string current and application. IMO’s flexibility means a safe, compliant solution for everything from small residential to multi-string commercial PV systems.

IMO DC Isolator Options:

• Current ratings:
  • 16 A → Ideal for single-string residential systems (Isc + 125% < 16 A)
  • 25 A → Suitable for hybrid/off-grid inverters with ~25 A inputs; balances safety and cost-effectiveness
  • 32 A → Designed for parallel string applications where combined current exceeds 16 A but remains under 32 A
• Pole configurations: 2-pole (single string), 4-pole (two strings, most common in UK), 6–8 pole (larger multi-string installs).
• Mounting options: Enclosed IP66/IP67 units for outdoor use, or panel/base-mount versions for integration in cabinets and string boxes.

➛ These ratings are the most common in UK solar installations and provide installers with confidence in both performance and compliance.

Trade Offer: Special Pricing on IMO Isolators

We currently provide exclusive trade discounts on IMO “True DC” isolators. 

➛ Explore DC Isolators

➛ Explore AC Isolators

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