A temporary distribution specification comes down to four decisions: the supply source and its capacity, the distribution hierarchy from incomer to work face, the protection regime matched to the actual load mix, and the tool-supply voltage. On UK sites the fourth is settled by convention, 110V centre-tapped-to-earth for portable tools, and it constrains what any monitoring layer can see. Get the first three wrong and the symptom is the same each time: a board replaced mid-project.
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UK practice supplies portable tools at 110V centre-tapped-to-earth, which holds the maximum voltage to earth at around 55V rather than 230V. That is the reduced-low-voltage principle behind HSE guidance for construction sites, and it is why a tool circuit sits downstream of a transformer and outside the reach of board-level instrumentation. Source: HSE, HSG141 Electrical safety on construction sites (second edition, 2023)
Key takeaways
| Topic | In brief |
|---|---|
| Source | Grid, generator or both. The choice changes fault level and therefore protection. |
| Hierarchy | Size the incomer on the project’s peak phase, not the sum of nameplates. |
| Protection | Device type follows the load mix, not the board rating. |
| Tool supply | 110V CTE for portable tools. Board instrumentation sits upstream of it. |
What decides the supply source?
Grid or generator is the first fork, and it changes everything downstream.
A grid connection generally gives a higher prospective fault level, which makes protective device operation more predictable and simplifies discrimination. Its problems are lead time and civils: the connection has to be built before it exists.
A generator is available immediately and moves with the work. Its constraint is that fault level is limited by the machine, which affects device selection and sensitivity, and its efficiency falls away sharply at low load. A genset run well below its rating burns fuel per kWh at a poor rate.
On longer projects the answer is frequently both: grid as the primary supply with generators for isolated areas or standby. That combination works but demands attention to the earthing arrangement and to transfer, and it is the configuration where informal specifications most often go wrong.
| Factor | Grid supply | Generator |
|---|---|---|
| Fault level | Higher, discrimination easier | Limited by machine, affects device choice |
| Availability | Depends on connection lead time | Immediate |
| Running cost | Usually lower per kWh | Fuel dependent |
| Efficiency at part load | Unaffected | Falls away below roughly a third of rating |
How should the distribution hierarchy be sized?
The recurring error is arithmetic. Nameplate ratings get added together and a board is ordered to match that total.
Real peak demand is driven by simultaneous starting rather than steady running. Motors, hoists, compressors, variable-speed drives and heaters draw substantially more at start than their nameplate suggests, and two coincident starts is the condition that finds an undersized incomer.
Three rules keep the sizing honest.
Size on the heaviest phase of the project, not the average. Frame stage is dominated by hoists and concrete work; fit-out by heating and drying. Whichever peak is higher sets the incomer.
Leave headroom. The saving from a tight specification disappears with the first nuisance trip that stops the site.
Count the winter. A project energised in summer and sized on summer demand meets a different reality when heating and drying load arrives in late autumn.
What protection regime does the load mix require?
Device selection follows the loads, not the board rating, and this is the part most often specified by habit.
Type A residual current devices are adequate for conventional loads. Variable-speed drives, welding sets and inverters can require Type B, because their fault current may contain a DC component that a Type A device will not reliably detect.
Getting this wrong shows up in one of two ways, both bad. Either the device fails to operate when it should, or it trips on inrush and the site learns to reset it reflexively, which is the more dangerous outcome because it trains people to treat protection as a nuisance.
Establish the load profile before ordering. BS 7671 Section 704 covers construction and demolition site installations and informs device selection; HSG141 is HSE guidance rather than law, and the statutory duty sits in the Electricity at Work Regulations 1989.
Why does 110V CTE constrain the monitoring design?
Portable tools on UK sites run at 110V centre-tapped-to-earth, separated from the site supply by a transformer. The centre tap holds maximum voltage to earth at around 55V, which is what protects the operator at the tool.
That transformer is also a boundary for instrumentation. Board-level monitoring sits on the supply side and sees supply-side conditions: load against capacity, phase balance, voltage, and the sequence preceding a trip. It does not see the tool circuit on the CTE side. Those faults remain with the inspection and test regime and the competent person.
This should be written into the specification rather than discovered during commissioning. A supplier claiming visibility of CTE tool circuits should be asked to demonstrate it on a live tool before that claim is relied upon.
Where does metering fit into the specification?
Metering and monitoring are separate jobs and conflating them inflates cost.
Monitoring is operational and continuous: is this board near capacity, are phases balanced, is voltage drifting. Its accuracy requirement is sufficient to act on.
Metering is commercial and periodic: how many kilowatt hours, to a standard someone will pay against. Where energy is recharged to a subcontractor or reported into a certification scheme, the instrument needs approval under Directive 2014/32/EU. On Spine, MID-approved energy metering is available as an optional accessory, fitted at the points where a billing boundary exists rather than everywhere.
Draw that boundary on the distribution schematic before energisation. Unmetered periods cannot be reconstructed afterwards.
Summary
Specifying temporary distribution is four decisions: supply source, hierarchy sized on the project’s heaviest phase, protection matched to the actual load mix, and 110V CTE for tools. The tool-supply convention also fixes where instrumentation can and cannot see, and that limit belongs in the specification from the outset. Metering for billing is a separate requirement from monitoring for operations. To work through a distribution specification, see worksite power supply or get in touch: sales@tsr-elsite.fi or +358 9 4555 588.
Frequently asked questions
Should a UK site run on grid supply or generators?
It depends on lead time and layout. Grid gives a higher fault level and lower running cost but has to be built; generators are immediate and mobile but limited in fault level and inefficient at low load. Long projects often use both, which requires care with earthing and transfer arrangements.
How do you size a temporary incomer correctly?
On the heaviest phase of the project, using real peak demand rather than the sum of nameplate ratings. Simultaneous starting drives the peak. Include the winter heating and drying load if the site is energised in summer.
When is a Type B RCD required instead of Type A?
When the load mix includes variable-speed drives, welding equipment or inverters, whose fault current can contain a DC component that a Type A device may not reliably detect. Conventional loads are adequately covered by Type A. Establish the load profile before ordering.
Is HSG141 a legal requirement?
No. HSG141 is HSE guidance and BS 7671 is a standard. The statutory duty sits in the Electricity at Work Regulations 1989, and the guidance and standards are the recognised means of meeting it.
Why is 110V used for tools instead of 230V?
Centre-tapped-to-earth supply holds the maximum voltage to earth at around 55V, roughly halving shock severity at the tool. It is the recognised approach for portable tools on UK construction sites. Exception: some fixed or specialist equipment runs at 230V or 400V with its own protection.
Can site monitoring see faults on the 110V tool circuits?
No. Board-level instrumentation sits on the supply side of the CTE transformer, so it covers supply-side conditions only. Tool-side faults stay with the inspection and test regime and the competent person’s examination.