Portable Generators on Site: Safe Connection, 110V CTE Supplies, Transfer Switches and G99 Compliance

Summary

Generators are ubiquitous on construction sites and in any situation where mains power is unavailable or unreliable. Despite their familiarity, they are a significant source of workplace injury — both from electrical faults and from carbon monoxide (CO) poisoning from petrol and diesel engines run in enclosed spaces.

The key safety principle with portable generators is that they are isolated systems. Their earthing arrangement is different from mains-supplied premises. When you connect tools or equipment to a generator, you must treat it as a standalone TT system and ensure appropriate protection is in place. Mixing generator and mains supplies without interlocking creates potentially lethal back-feed hazards.

On construction sites, the BS EN 60309 (formerly BS 4343) plug and socket standard specifies which colours and keyway positions correspond to which voltages. The 110V yellow CTE supply is the standard for portable hand tools on site — it limits the shock voltage to 55V if the earth is lost, substantially reducing fatality risk. Understanding when 110V is required, and when 230V tools can be used, is a core competency for site electricians and construction managers.

Key Facts

• Generator earthing — Portable generators must have their own independent earth electrode; they cannot rely on the premises earthing system
• TT system — Most generator supplies are TT (terra-terra): both source and earth are local; RCD protection is essential
• 110V CTE — Centre-tapped earth at 55V to earth; standard for handheld and hand-guided tools on construction sites per HSE guidance and BS 7671
• Yellow plugs/sockets — BS EN 60309 110V: yellow, 4-hour keyway position (see table below)
• Blue plugs — BS EN 60309 230V 50Hz single phase: blue
• Red plugs — BS EN 60309 400V 3-phase: red
• Transfer switch requirement — Any generator connected to existing fixed wiring must have an interlocked transfer switch (mechanical or electrical interlock) to prevent parallel operation with mains
• Regulation 22 — Electricity Safety, Quality and Continuity Regulations 2002 Reg 22: no private generating set may be connected to the public network without consent of the distributor
• G99 — Engineering Recommendation G99: connection of generating plant to the distribution network — applies to generators that can export to the grid
• CO risk — Petrol generators in an enclosed space can generate lethal CO levels within minutes; mandatory minimum 1m clearance from openings, and never indoors
• BS 7375:2010 — Distribution of electricity on construction and demolition sites
• Minimum RCD — 30mA RCD required on all 230V socket circuits from a generator per BS 7671

Quick Reference Table

Detailed Guidance

Generator Earthing on Construction Sites

When a portable generator is used as a standalone supply, it must have its own earth electrode. Do not assume the generator frame or fuel tank provides adequate earthing — it does not.

Earthing arrangement:

• Drive a temporary earth electrode: copper-clad steel earth rod (≥1.2m length) into ground within 2m of generator
• Connect generator neutral (if star-wound) and generator frame to electrode using green/yellow cable ≥6mm²
• Test electrode resistance: target <200Ω; ≤100Ω preferred for effective RCD operation

Why TT earthing matters: In a TT system, fault current must flow back through the earth electrode. If electrode resistance is high, fault current may be low enough that fuses/MCBs don't trip. RCDs are essential because they detect the small differential current (30mA) regardless of electrode resistance. Always fit a 30mA RCD at the generator output or at the first distribution board.

Temporary distribution boards:

• Use a properly rated site distribution board (BS EN 61439-4: Particular requirements for assemblies for construction sites — ASCs)
• Board must be IP44 minimum for outdoor use (IP55 in wet conditions)
• Each socket outlet must have individual MCB and RCD protection
• Board must be labelled with generator source, voltage, and date of test

110V CTE: The Site Standard

The 110V centre-tapped earth supply is the HSE-recommended standard for all handheld and hand-guided tools used on outdoor or construction sites. The principle:

• Transformer secondary is centre-tapped and the mid-point is earthed
• Each conductor is 55V to earth
• Maximum shock voltage if either conductor is touched = 55V (rather than 230V)
• At 55V, cardiac fibrillation is possible but fatality probability is substantially lower than at 230V

HSE guidance (GS37): All 230V portable and hand-held tools should be replaced by 110V CTE tools where practicable on construction sites. This is not a legal absolute requirement but is the expected standard.

When 230V tools are acceptable:

• Fixed tools in a securely enclosed site workshop with GFCI/RCD protection and controlled access
• Office equipment in site office (separate protected space)
• Equipment that is not available in 110V format (some specialist equipment)

Never use a 230V tool with a 110V site supply by adapting the plug. This violates BS EN 60309 colour coding and creates a safety hazard. Buy the 110V version or use proper step-up transformer arrangements.

Connecting a Generator to Fixed Premises Wiring

The most dangerous scenario is connecting a generator to a property's fixed wiring without proper interlocking. This creates a back-feed hazard: if the mains supply is restored while the generator is running, the generator can:

• Energise the network cables outside the property (risk to DNO engineers working on the line)
• Suffer immediate damage or destruction from phase mismatch
• Cause electric shock to anyone touching the presumed-dead cables

Transfer switch requirement:

A transfer switch mechanically or electrically prevents simultaneous connection of both mains and generator. Options:

1. Changeover switch (manual): A 3-position rotary switch: Mains / Off / Generator. Cannot be in both positions simultaneously. Simple and reliable; suitable for most temporary arrangements.

2. Automatic Transfer Switch (ATS): Monitors mains supply; on loss of mains, starts generator (if auto-start fitted) and switches over automatically; on mains restoration, transfers back after a stabilisation delay. Used in critical supply applications, UPS systems, data centres.

3. Inlet socket with interlocked RCD: Some panel-mounted generator inlet sockets (Commando type) have built-in interlocking to disconnect the mains MCB when the inlet socket is occupied.

Notification to DNO:

• Under ESQCR 2002 Regulation 22, any generator that can be operated in parallel with the distribution network requires prior consent from the distributor
• For temporary backup generators that are always isolated from the network (via transfer switch): notification generally not required
• For generators capable of export (or parallel operation): G99 application required

G99 and Export-Capable Generators

G98 and G99 are Engineering Recommendations from Energy Networks Association (ENA):

• G98: Single-phase generating plant ≤16A (3.68kW) — simplified registration; no application required; installer registers post-connection
• G99: All other generating plant; requires application and approval from the Distribution Network Operator (DNO) before connection
• Purpose: Ensures that generator frequency, voltage, and anti-islanding protection are compatible with network operation

For portable site generators: These are almost never G99-relevant because they are isolated from the public network by the transfer switch. G99 becomes relevant when a diesel generator is installed as a permanent facility and is capable of exporting.

Anti-islanding protection (G99): All grid-connected generators must include anti-islanding protection that disconnects the generator from the network if the network supply is lost. This prevents energising a network section that operators believe is dead.

CO Safety with Petrol and Diesel Generators

Carbon monoxide is the leading cause of generator-related deaths. Petrol engines in particular produce large volumes of CO.

Rules:

• Never run a generator indoors, in a garage, in a tent, or near an open window or door
• Maintain minimum 1m clearance from building openings (doors, windows, vents)
• Never run in an enclosed or partially enclosed space
• Fit battery-powered CO alarms if generator will be used near living or working spaces
• CO has no smell; symptoms (headache, nausea, confusion) can progress to death rapidly

HSE guidance: CO from generators causes approximately 20 deaths in the UK each year, mostly in construction, events, and domestic emergency use.

Generator Maintenance for Site Use

Frequently Asked Questions

Can I plug a generator directly into a standard 13A socket in my house?

No. You must use an interlocked transfer switch that disconnects the mains supply before the generator is connected. Using a regular extension lead from a generator plugged into a house socket creates a parallel supply arrangement, and if the mains is live when you do this, you can damage the generator, blow MCBs, and potentially back-feed the street. Some people use a "suicide lead" (male plug at both ends) — this is illegal, immediately dangerous, and has caused multiple deaths.

Does a portable generator need PAT testing?

If the generator is used for work and is employer's equipment, it is subject to the Provision and Use of Work Equipment Regulations 1998 (PUWER) and the Electricity at Work Regulations 1989. PAT testing (formal in-service inspection and testing) is one way to demonstrate compliance, though it is not the only approach. Frequency depends on the risk assessment — a generator on a construction site might be inspected formally every 3 months; a standby generator in a dry office environment might be tested annually.

What size generator do I need for a standard construction site?

Calculate total connected load: sum the wattage of all tools and equipment that might run simultaneously. Add 20% contingency. Divide by power factor (0.8 typical for mixed loads): kVA = kW ÷ 0.8. A 5kVA generator (4kW output) typically handles: one 2.2kW angle grinder, site lighting, radio, battery chargers. A 10kVA handles larger tools in combination. Avoid loading a generator above 80% of nameplate rating continuously.

What is the difference between standby power and prime power ratings?

Prime power — the maximum load the generator can sustain for unlimited hours (continuous operation). This is the rating you should use for site or backup use.

Standby power — the maximum load for emergency use (typically 10% above prime power) for up to 500 hours per year with average load factor 70%. Do not size for standby rating if the generator will run continuously.

Regulations & Standards

• Electricity Safety, Quality and Continuity Regulations 2002 (SI 2002/2665) — Regulation 22: restriction on connection to public network

• Electricity at Work Regulations 1989 — Duties for electrical safety in workplace

• BS 7671:2018+A2:2022 — Wiring Regulations: generator earthing, RCD requirements

• BS 7375:2010 — Distribution of electricity on construction and demolition sites

• BS EN 61439-4 — Assemblies for construction sites (ASCs) standard

• BS EN 60309-2 — Plugs, socket-outlets and couplers for industrial purposes (site plugs)

• Engineering Recommendation G99 — Connection of generating plant to the distribution system

• HSE GS37 — Electrical safety in arc welding (references 110V CTE principle)

• PUWER 1998 — Provision and Use of Work Equipment Regulations

• HSE Construction Electrical Safety — Guidance on temporary supplies and generators

• Energy Networks Association G99 — G99 requirements for generator connections

• HSE Carbon Monoxide from Generators — CO risk and prevention guidance

• consumer units — Fixed installation context for generator transfer switching

• cable sizing — Cable sizing for generator supply cables

• part p notifications — Notifiable work when installing generator transfer switches