but also an Earth Terminal, which seems strange since these alarms are Class II Apparatus

And a very good thing too - since BS 7671 requires (in domestics at least) a c.p.c. to run to and terminated at each point in wiring. Much better than trying to squeeze in a floating terminal into a space whose designers hadn't considered it.

   - Andy.

AJJewsbury said:

And a very good thing too - since BS 7671 requires (in domestics at least) a c.p.c. to run to and terminated at each point in wiring.

At least in 17th and 18th Edition, only where ADS is used (Regulation 411.3.1.1 in Section 411).

It is therefore not a specific requirement for PELV (which is Section 414).

I suppose PELV systems could have an effective PEM conductor in theory ... for example a combined '0 V and cpc or ground'. In the past I've also in the past been witness to debate on whether this is permitted in the UK in consumer's installations (because of ESQCR). It has, arguably, certainly been used, for example in 3-wire unscreened EIA-232 (RS232) where PG and GND are bonded at both ends, or where GND is connected to PG at one end and PG is not used at the other. Effective PEM conductors may also appear in parts of PELV control systems, but often the wiring is enclosed in exposed-conductive-parts or metallic containment, which are connected to PE (or in machinery, the protective bonding circuit, which is the same thing).

At least in 17th and 18th Edition, only where ADS is used (Regulation 411.3.1.1 in Section 411).

Also 412.2.3.2 (double or reinforced insulation).

And Clive's link seems to be to a typical 230V powered variety ... so not PELV either (at least not the L/N/PE bit...I suppose the interlink might be).

   - Andy.

AJJewsbury said:

Also 412.2.3.2 (double or reinforced insulation).

Well, not necessarily where 412.1.2 applies, although as the note to 412.2.3.2 says, the provision is to ensure ADS can be used in the circuit if needed, as per the note.

In PELV systems, earthing of exposed-conductive-parts of circuits and equipment is not always necessary, but may be provided (414.4.1).

AJJewsbury said:

And Clive's link seems to be to a typical 230V powered variety ... so not PELV either (at least not the L/N/PE bit...I suppose the interlink might be).

Agreed.

AncientMariner said:

Like our existing alarms, there are three terminals, Live, Neutral and Interconnect  on a plug in connector, but also an Earth Terminal, which seems strange since these alarms are Class II Apparatus.

I wonder if the PE connection is used for functional purposes in the device? Only the manufacturer would know.

gkenyon said:

I wonder if the PE connection is used for functional purposes in the device? Only the manufacturer would know.

The instructions for an Aico Ei3024 alarm make it perfectly clear (their emphasis): "check that the Interconnect wire is NOT connected to Live, Neutral or Earth. Do not use the Earth wire for the Interconnect line."

It continues: "N.B. The Alarm does not need to be earthed. However the terminal marked [symbol for earth] is provided for the convenience of the installer so that any copper Earth wire or cable coloured green and yellow, can be safely terminated."

gkenyon said:

I wonder if the PE connection is used for functional purposes in the device? Only the manufacturer would know.

The instructions for an Aico Ei3024 alarm make it perfectly clear (their emphasis): "check that the Interconnect wire is NOT connected to Live, Neutral or Earth. Do not use the Earth wire for the Interconnect line."

It continues: "N.B. The Alarm does not need to be earthed. However the terminal marked [symbol for earth] is provided for the convenience of the installer so that any copper Earth wire or cable coloured green and yellow, can be safely terminated."

which suggest its just an anchor to avoid  a loose end. Albeit it implies one that can sense the colour and presence of earth sleeving which I doubt.
Flippancy  aside, similar models I have looked inside, signal the interlink 'alarm' condition by placing the battery voltage (so 6-9V) between neutral and the interlink wire. They really don't appreciate it if one alarm has a L-N reversed supply as then the interlink electronics sees the full mains, instead of some volts DC, and fails, quite impressively.
Mike.

BS 5446 series covers the standards

BS 5446-1: This part specifies requirements and test methods for smoke alarms, including those that use optical, transmitted light, or ionization technologies. 

BS 5446-2: This part defines the specifications for heat alarms, covering requirements and testing for devices intended for permanent installation in dwellings. 

BS 5446-3: This standard focuses on alarm systems for deaf and hard-of-hearing people, specifying requirements for fire and carbon monoxide alarms and associated warning devices like visual or low-frequency sounders. 

BS 5446-4: This part specifies requirements for multi-sensor fire alarms that use a combination of smoke, heat, and/or carbon monoxide sensors

mapj1 said:

Flippancy  aside, similar models I have looked inside, signal the interlink 'alarm' condition by placing the battery voltage (so 6-9V) between neutral and the interlink wire. They really don't appreciate it if one alarm has a L-N reversed supply as then the interlink electronics sees the full mains, instead of some volts DC, and fails, quite impressively.

Is it possible that BS 5446 series needs to be updated to cater for the above scenario?

This MAY have some overlap with a discussion I previously created titled

Does BS 5839-6 or is there a product standard for UK smoke/heat/multi sensor alarm that covers the following?

engx.theiet.org/.../does-bs-5839-6-or-is-there-a-product-standard-for-uk-smoke-heat-multi-sensor-alarm-that-covers-the-following

Chris Pearson said:

It continues: "N.B. The Alarm does not need to be earthed. However the terminal marked [symbol for earth] is provided for the convenience of the installer so that any copper Earth wire or cable coloured green and yellow, can be safely terminated."

THank you, so for ease of conformity to 412.2.3.2 (based on Note to that Regulation).