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Topic Title: Reduced Low Voltage Systems
Topic Summary: Section 411.8 of the IET Wiring Regulations
Created On: 10 October 2018 09:06 AM
Status: Read Only
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 10 October 2018 09:06 AM
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robertchilds

Posts: 17
Joined: 16 July 2014

As part of a fixed wiring inspection report we have been requested to provide double pole protection in accordance with Section 411.8 of the IET Wiring Regulations to the primary side of the 110V transformer.

Am I correct in saying that this regulation refers only to the secondary side of the transformers i.e the 110V Reduced Low Voltage Circuits ?
 10 October 2018 09:26 AM
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Zoomup

Posts: 5464
Joined: 20 February 2014

411.8.3 requires fault protection by automatic disconnection of supply being provided by means of an overcurrent protective device in each line conductor or by an RCD.... This must apply to the secondary of the source transformer to make any sense, as the regulation later refers to earth fault loop impedance and disconnection times, which must apply to the secondary supply and final circuits loads.

Z.
 10 October 2018 09:32 AM
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robertchilds

Posts: 17
Joined: 16 July 2014

Many thanks for the quick reply Z
 10 October 2018 12:37 PM
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wliptrot

Posts: 120
Joined: 30 June 2003

Have you been requested to do work in response to an EICR report? It wasn't clear to me....

Is the 110V Transformer part of a small 110V distribution system? A safety isolation type? Is it Centre tapped secondary...curious really.

I would be expecting the Primary side to have protection in the Line conductor only.

And I would expect secondary protection to disconnect both lines, whether centre tapped or not.
 10 October 2018 03:21 PM
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broadgage

Posts: 3023
Joined: 07 August 2007

Presuming a standard single phase 230/240 volt input from UK style mains, then the primary only needs OCPD in the line conductor, just like any other single phase load.

I would only consider double pole primary protection to be required in the unlikely event that there is something unusual about the supply.
Within a caravan or similar moveable unit.
Liable to be taken overseas.
Likely to be supplied from a small portable generator.

The secondary needs double pole protection if it has an earthed centre tap, or if it is floating.
Single pole protection would be acceptable in the line wire only if one end of the secondary is earthed, unlikely in the UK.
 15 October 2018 08:16 AM
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robertchilds

Posts: 17
Joined: 16 July 2014

wliptrot,

Yes, we have been asked to do work in response to a fixed wiring inspection. There is nothing special about the 110V transformer, it is centre tapped to earth and the primary is standard single phase 230/240 volt supply.
The primary supply is protected on the Line conductor and the secondary is protected on both lines.
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