Bad idea??

[studentspark]

Odd one for me

Need to install some emergency lighting
No perm live at lights
Running new cable has problems

Possible to take a feed from a socket circuit to power emergency lights, but this is a different circuit to the lighting so no good.

Looking into possible solutions and thinking of relays.
Now is this do-able, possible or a non starter ?

Power a relay coil from the lighting MCB, and straight on to power general lighting. Coil will close..
Feed the NO contacts with the feed from the socket circuit MCB, so the lighting circuit closes the contacts and sends power onto the socket.

The socket circuit then powers the emergency lighting.
Should lighting fail, this will cut power to the coil, close to NC, cut power to the socket and emergency lighting will come on.

I've attached a drawing and would appreciate any thoughts.



Thanks View media item 85221

 

[deadshort]

Bad idea, why not maintained ? and replace the switches with keyswitch ?

Regards,

DS

 

[studentspark]

only got a switched live

 

[deadshort]

If you replace the switch with a key switch (which is switched on during normal day to day use )that provides your perm live for the charger, and to test you switch off the power ?

If you install emergency lighting and deviate from the "norm"
you are liable if the system fails to operate due to your design.



Kind Regards,

DS

 

[EFLImpudence]

Studentspark: Haven't you got it all the wrong way round?

 

[DaftPunk]

How about making the existing wiring to the lights a permanent feed and fitting PIRs or microwave detectors to the existing fittings?

 

[JohnW2]

Studentspark: Haven't you got it all the wrong way round?

Unless there is some confusion about the NC/NO terminology, it would seem that he has (plus some other problems) ...

Power a relay coil from the lighting MCB, and straight on to power general lighting. Coil will close.. Feed the NO contacts with the feed from the socket circuit MCB, so the lighting circuit closes the contacts and sends power onto the socket.The socket circuit then powers the emergency lighting.

All true, but that's surely not what you want. You surely want the emergency lighting to be powered if the lighting circuit fails, not when it's live? You therefore would need to connect the sockets to to NC (closed when coil not energised) contacts of the relay?

Should lighting fail, this will cut power to the coil, close to NC, cut power to the socket and emergency lighting will come on.

No, as above, with what you are suggesting, the power to socket and emergency lighting would go off when the lighting power failed and hence relay coil was unenergised.

After one has moved the sockets circuit to the NC contacts, I see a few problems. Firstly, if you interpose the relay (and it would have to be a hefty one, if it was switching a 32A circuit) at the MCB end of the sockets circuit, it would mean that the entire sockets circuit would nearly always be dead, becoming live only if the lighting MCB operated whilst power was still available for the sockets circuit! Furthermore, as a more minor issue, if it were a ring circuit, you'd have to have the relay upstream (MCB side) of where the circuit split into the two arms of the ring, turning it into a 'lollipop' circuit.

If you were taking this sort of approach, surely far better to take a spur (if a ring) or branch (if a radial) from the sockets circuit MCB and take that through a 3A (or 1A!) FCU and relay (much less hefty relay then needed if downstream of the FCU) and use this as a dedicated feed for your emergency light (without the need for a socket), leaving the existing sockets circuit intact (and not fed through a hefty relay!).

Secondly, it would be a pretty restricted sort of 'emergency lighting' (in comparison with having a battery backed-up system). The emergency lighting would only come on if the lighting circuit power failed whilst the sockets circuit was still live - which, in practice, means that it would only ever come on if the lighting MCB operated.

Kind Regards, John

 

[JohnW2]

How about making the existing wiring to the lights a permanent feed and fitting PIRs or microwave detectors to the existing fittings?

A clever idea - but if loss of lighting is deemed to present a degree of hazard such as to require this 'emergency lighting', it probably would not be acceptable to have the lights going out if a person stood still for too long!

Kind Regards, John

 

[bernardgreen]

John has made point that emergency lighting must be able to operate correctly during a total loss of electrical power and thus an emergency supply has to be part of the system.

Replacing some of the light fittings along exit routes with fittings that have internal batteries is the simple way to achieve this.

 

[studentspark]

Oh, have I got the workings of a relay wrong

1-4 NC
8-5 NC
1-3 NO
8-6 NO
1-8 com
2-7 coil

Now I thought if the coil was not powered, continuity would be between

1-4 and 8-5 (NC)

In my scenario I was not using these points. so no power to socket

I thought with the coil powered via the lighting circuit, the continuity would between 1-3 and 8-6 (NO)

So with the lighting MCB on, coil energised to the NO.

Lighting MCB off springs back to NC

The socket would be powered via 1-3 and 6-8 (Double pole switching of Live and neutral)

Now if the lighting MCB was off and sending no power to the coil, this would snap back to 1-4 and 8-5 and hence no power to the socket.

So the feed to the Emergency Light, which gets its power from the socket would also be cut, and the battery back up would light the emergency light.

The socket would always have power whilst the coil is energised by the lighting circuit.
The socket would only lose power if the lighting circuit failed, and power was cut to the coil, snapping the relay back to 1-4 and 8-5.



The socket circuit is only two single sockets for the cleaners hoover, this can be fused down so the relay does not have to be so hefty.


I like the PIR idea.

Dead short - I can't quite follow what you mean

 

[deadshort]

Hi, sorry i have just realised ! you are not actually using 'emergency light fitting with battery back up? I would not class your system as emergency lighting.

You are infact,switching on another light source via the N/O relay. If you have a power failure to the supply you have no light,that to me, is not emergency lighting.

Kind regards,

DS

 

[studentspark]

When I say emergency light, I mean one that sits there all day OFF but powered (little green light) and if supply is cut, it comes on via internal battery back up.
Non maintained.

 

[deadshort]

Thanks for the clarification.

DS

 

[JohnW2]

Oh, have I got the workings of a relay wrong
1-4 NC ... 8-5 NC ... 1-3 NO ... 8-6 NO ... 1-8 com ... 2-7 coil
Now I thought if the coil was not powered, continuity would be between
1-4 and 8-5 (NC)
In my scenario I was not using these points. so no power to socket
I thought with the coil powered via the lighting circuit, the continuity would between 1-3 and 8-6 (NO)
So with the lighting MCB on, coil energised to the NO.
Lighting MCB off springs back to NC
The socket would be powered via 1-3 and 6-8 (Double pole switching of Live and neutral)
Now if the lighting MCB was off and sending no power to the coil, this would snap back to 1-4 and 8-5 and hence no power to the socket.

That is all correct!

So the feed to the Emergency Light, which gets its power from the socket would also be cut, and the battery back up would light the emergency light.

Ah. I misunderstood!! Like deadshort, I thought your 'emergency light' was just a lamp, not a proper ('non-maintained') emergency light with battery back-up (implicitly containing a second relay, which effectively reverses the logic of when the emergency light comes on). If that's what it is, then I agree that your proposed system would work.

The socket would always have power whilst the coil is energised by the lighting circuit. The socket would only lose power if the lighting circuit failed, and power was cut to the coil, snapping the relay back to 1-4 and 8-5.

Yep, now that I understand what your proposed 'emergency light' is, I agree.

The socket circuit is only two single sockets for the cleaners hoover, this can be fused down so the relay does not have to be so hefty.

Fair enough, but I'd still personally be inclined to take a separate spur/branch (with FCU) from the MCB (or from one of the sockets) for your relay and emergency light, so that your relay wasn't switching the feed to the sockets at all.

Kind Regards, John