Emergency lighting questions

[RF Lighting]

Hello all.

I’ve been asked to take a look at a central battery emergency lighting system which has stopped working.

It’s been in a good few years (45+!) and is in need of a little tlc.

This is the main control cabinet.

I have traced the initial fault to one of the changeover relays in the top right of the cabinet, which is an easy enough fix.

While I was there, I was a little concerned by a few things.

There are a couple of capacitors which appear to have collapsed and I guess they are no longer functioning. What would they be in the circuit for? I think they are in the rectified AC supply for when the lights are transformer supplied in maintained mode.

Also I’m concerned by the output protection being provided by single pole mains circuit breakers on a 24V DC supply. Is this arrangement allowed, and more to the point, will they even operate in the event of a fault?

Feel free to point out anything else I’ve missed too.

 

[333rocky333]

I am no expert, but the ones i have seen are usually Hrc fused in both the red and black outputs for each outgoing circuit, I once saw one that had Double pole Mcbs fitted, but were marked as 110 volt and someone said at the time they were ok up to 110volt and i think it may be in the merlin gerin technical info, at the time the current merlin gerin style were the same as the Mk ones you appear to have.
As for the caps, not sure them lights look original, i wonder if they had ballasts in at some time and the caps were just to improve the lagging power factor

 

[JohnW2]

As for the caps, not sure them lights look original, i wonder if they had ballasts in at some time and the caps were just to improve the lagging power factor

They have far far higher a capacitance (6,800μF) than would be necessary for PF correction unless the ballast were the size of a bus. Also, being 63V rated (and with no mention of an AC rating), I would be presume that they would be working on the 24A DC circuitry. Might they perhaps be 'reservoir capacitors' in the charging circuit?

Kind Regards, John

 

[winston1]

Those capacitors have not collapsed, that is the way they were made. Anyway capacitors tend to swell not collapse when they fail. Worthwhile testing them though.
MCBs are current operated devices and I see no reason that will not operate when used on a 24 volt supply.

 

[Adam_151]

MCBs are current operated devices and I see no reason that will not operate when used on a 24 volt supply.

Thats true, however its the DC bit that worries me, the arc might not extinguish properly, might ne worth a call to MK technical?

I've sucesfully used noral MCBs on a 24v AC supply, hoever DC is a diffeenr beast

 

[JohnW2]

MCBs are current operated devices and I see no reason that will not operate when used on a 24 volt supply.

It's not the 'voltage' (about which a basic MCB knows nothing) or the current which is the potential issue, but the fact that it is DC. With a potentially high DC current flowing through it ('in the same direction') over a long period of time, there is a risk that everything around will get magnetised, which could adversely affect its function - plus the DC arcing problem when it breaks a DC circuit. I believe that breakers designed specifically for DC are available.

Kind Regards, John
Edit: Hadn't seen Adam's post!

 

[echoes]

The size of the capacitors does suggest they are being used as smoothing capacitors for the rectified d.c. Presumably when the mains is on, the d.c. keeps the relays energised, and would need a smoothed voltage across the coils to stop them buzzing at 100 Hz.

Edit: on closer look, relays have a.c. coils (?), so caps just for smoothing.

Are those 2 black things bottom left the rectifiers?
Edit 2: No, of course not. Spotted the bridge rectifier now!

 

[SUNRAY]

Replace with proper DC MCB's and know it is done properly.
A colleague used a load of AC MCB's in a battery backed DC system and found they tripped at a lower current. This % error was not consistent across different rated MCB's. On 24V AC, standard MCB's are just fine.

 

[RF Lighting]

Since posting this I've found proper DC circuit breakers which I will be fitting in place of the existing.

The relays are fed from the 230V AC supply and are normally energised to keep the boxed exit lights lit on the rectified mains supply, and switch all lights to the battery supply when the mains fails.

I'm back at this site next week so I'll investigate further and draw up a schematic to see where in the circuit the capacitors are, but from memory they are part of the DC circuitry and are in circuit both on the transformer and battery supplies.

The client is looking for a reasonably cheap fix as this system will be removed once the building is rewired and self contained locally powered units are installed, but this will be a good few years down the line until each section of the building is refurbished.

 

[JohnW2]

I'm back at this site next week ....

That sounds like 'treble time' to me - I didn't think any electricians were working next week

... so I'll investigate further and draw up a schematic to see where in the circuit the capacitors are, but from memory they are part of the DC circuitry and are in circuit both on the transformer and battery supplies.

As I said, I cannot believe it could be otherwise, with a 63V rating, and no mention of an AC rating, they must be in a DC ELV circuit. I can't think what (given their capacitance) what they could be other than 'reservoir'/'smoothing' capacitors following the rectifier.

Kind Regards, John

 

[333rocky333]

Also bear in mind the regs have changed regarding arrow down signs and they may not comply.
Arrow down signs really are on the way out

 

[SUNRAY]

Also bear in mind the regs have changed regarding arrow down signs and they may not comply.
Arrow down signs really are on the way out

It seems only 5 minutes ago we changed all the up arrows to down arrows...


BTW that cabinet is in quite good condition, many of these systems are really badly neglected, I'll make a bet those mcb's were all fuses originally and wired with standard 7 or 3/0.029 and the trunking is not original.
The left cap is directly across the dc out of the bridge rectifier so def a smoothing/reservoir. Can't follow the wiring for the right.

 

[ban-all-sheds]

It seems only 5 minutes ago we changed all the up arrows to down arrows...

It took me quite some time of driving in France before I realised that a direction signs on buildings at crossroads in towns and villages didn't mean "turn left" or "turn right" - they were in fact pointing at the road you needed to take, i.e. "straight on - this one here is the road you want".

There are no universally accepted conventions for what arrows mean, except on direction signs in hospitals - there the people who put them up accept that they mean something different to what every other person in the world thinks they must mean.

 

[SUNRAY]

It took me quite some time of driving in France before I realised that a direction signs on buildings at crossroads in towns and villages didn't mean "turn left" or "turn right" - they were in fact pointing at the road you needed to take, i.e. "straight on - this one here is the road you want".

There are no universally accepted conventions for what arrows mean, except on direction signs in hospitals - there the people who put them up accept that they mean something different to what every other person in the world thinks they must mean.

I thought we changed our emergency exit sign to harmonise with EU.

 

[Adam_151]

Did you order the DC breakers?, Rob. Just curious as to what they cost... knowing how schneider price their gear its not going to be cheap! Think I'd have gone with a few slyd-lok or red spot carriers with BS88 fuses in... which is what it looks like someone may have previously removed to fit those MK mcbs....!