ELE/General/TBT-ELE-005

RCD Protection and 110 Volt Use

Electrical Safety › General › RCD Protection and 110 Volt Use

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110 volt supply through a centre-tapped earth transformer is the standard for portable tools on UK construction sites.
The CTE system limits the maximum voltage to earth to 55 volts, significantly reducing the severity of electric shock.
Residual current devices detect earth faults and disconnect the supply in milliseconds, preventing prolonged shock.
30mA RCDs must protect all 230 volt circuits used on construction sites where 110 volt alternatives are not available.
RCDs must be tested by pressing the test button daily before use to confirm they trip correctly and reset properly.
RCDs protect against earth faults but do not protect against all types of electric shock, such as line-to-line contact.
110 volt transformers must be inspected regularly and PAT tested at the required intervals for the site environment.
Damaged 110 volt leads with exposed cores or cracked plugs must be taken out of service immediately and replaced.
Extension leads should be fully unwound when in use to prevent overheating from the coil effect in the cable.
The combination of 110 volt supply and RCD protection provides the safest practical electrical supply for construction sites.

Reduced shock severity	110 volt CTE limits the shock to 55 volts maximum to earth — this voltage is far less likely to cause a fatal outcome.
Millisecond protection	A 30mA RCD disconnects in under 40 milliseconds — fast enough to prevent the sustained current flow that causes cardiac arrest.
Daily testing	An RCD that does not trip when tested may not trip during a real fault — daily testing is the only way to confirm it works.

Don't

Use 110 volt tools through a CTE transformer for all portable equipment on site.
Install 30mA RCD protection on all 230 volt circuits used on the construction site.
Test every RCD daily by pressing the test button before starting work.
Inspect 110 volt leads, plugs, and transformer units before each use.
Replace damaged leads with exposed cores or cracked plugs immediately.
Fully unwind extension leads when in use to prevent overheating in the coil.
PAT test all 110 volt equipment at the intervals required for the site environment.
Use 230 volt supply only where no 110 volt alternative exists for the task.
Ensure transformers are positioned safely away from water and traffic routes.
Report any RCD that fails to trip when the test button is pressed.

DON'T use 230 volt portable tools on site when 110 volt alternatives are available.
DON'T use 230 volt circuits without 30mA RCD protection on construction sites.
DON'T skip the daily RCD test — an untested device may not protect you.
DON'T use 110 volt leads with visible damage, exposed wiring, or cracked plugs.
DON'T continue using damaged electrical leads — take them out of service immediately.
DON'T leave extension leads coiled when carrying current — unwind them fully.
DON'T use equipment with expired or missing PAT test labels.
DON'T assume 110 volts is completely safe — it can still cause injury in wet conditions.
DON'T place transformers in standing water or on wet, unprotected ground.
DON'T reset an RCD that keeps tripping — investigate the fault before resetting.

See also: Electrical Safety Awareness | Temporary Electrical Installations

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