Dc Through Ac Mcb's

[Lurch]

From my reading of the OP the DB is fed from AC or DC via a set of contactors so the switches will come into it.

 

[ProDave]

I think the one thing we all agree on, is the present system is a bodge with lots of problems.

 

[Andy™]

also, shouldnt central battery systems be wired in pyro / FP?

 

[Lurch]

I think this whole scheme needs looking at. There are a lot of lights here and they are specially ordering LED fittings so that they will work with a weird non-compliant system that has its faults. Surely the central battery system is not cost effective compared to just replacing some of the existing fittings with standard emergency packs once the TCO is all worked out?

 

[Essex1]

also, shouldnt central battery systems be wired in pyro / FP?

Not sure if FP is allowed.  Pyro yes.  

 

[Sidewinder]

It's only the current carrying capacity of the switches that will matter.

If & only if the standby supplies are fed to the luminaires through the functional switches.

Which, to be compliant they must never be.

The FP/Pyro argument will come down to client specifications.

As long as you have 3hr fire rating on the standby supplies then that is fine, or less if the luminaires are shorter rated.

Yes it is either a dangerous bodge, or something has not been explained to the OP correctly.

 

[pickleye]

Hi all, thanks for the replies.

There are no functional switches on the system as its all fired from a panel at reception. Power to the contactor coil.

This connects the corridor board to the AC supply. If you knock off the AC, another clunk is heard and then DC is present on the breaker.

I'm limited to what I can check during the week, site spark doesn't see a problem, been ok for 40 years.

It's wired in singles too, not pyro, so yes good point. Apparently that was looked at when specking the lights, but it was decided to leave it.

 

[Essex1]

Has this been actually designed?

 

[pickleye]

I'd guess yes, but by a milkman.

 

[Southcoast2015]

If there is an issue with the DC running through MCB and any other switch gear, Could you not use an inverter to change it to AC before it enters the MCB's.

Of course this will only work if the emergency circuit in the lights will run on AC.

 

[Lurch]

Of course this will only work if the emergency circuit in the lights will run on AC.

It seems that there is only 1 type of fitting with 1 circuit in it so it runs on AC under normal conditions and DC in a power failure.

 

[binky]

it's an old fashiouned centralised emergency battery system - I've only ever seen the remains of 1, far easier as said above to do away with it.

As for 6kV MCBs, the  fact the incoming is 10kV is irrelevent, the individual ccts protected by individual MCBs and related maximum fault currents of those ccts is what actually matters. ie MCBs protect what's downstream of them, and that PFC is what relates to the 6kV rating. This subject has been discussed on here before.

 

[pickleye]

I'd love to do away with it or send it through an inverter, but those decisions are not mine to make.

The battery unit was replaced in 2013 ffs.

 

[Essex1]

In some areas UPS systems are the only thing allowed to meet the required protection.

 

[HandySparks]

it's an old fashiouned centralised emergency battery system - I've only ever seen the remains of 1, far easier as said above to do away with it.

As for 6kV MCBs, the  fact the incoming is 10kV is irrelevent, the individual ccts protected by individual MCBs and related maximum fault currents of those ccts is what actually matters. ie MCBs protect what's downstream of them, and that PFC is what relates to the 6kV rating. This subject has been discussed on here before.

Assuming you mean kA, not kV.

The maximum fault current for the circuit will be the PFC at the DB (or near enough) if the fault occurs close to the DB.

 

[Sidewinder]

In some areas UPS systems are the only thing allowed to meet the required protection.

However, a UPS is a different animal to a central battery system.

 

[binky]

Assuming you mean kA, not kV.

The maximum fault current for the circuit will be the PFC at the DB (or near enough) if the fault occurs close to the DB.

Nope the max PFC for the cct is just after the MCB, to get near 6Ka, you need to dead short the cct about 1mm after the MCB to get almost zero resistance - generally physically impossible

 

[pickleye]

We've got pfc of approaching 10k on 6k MCB's. Old install in metal containment. The decision was made to replace all effected with 10k variety.

Site engineers argued it's not been a problem in 40 odd years. Counter argument - yes it's 40

odd years old, time to expect issues.

No one would sign off on their argument to leave them alone, so there coming out.

We've got 12kA on some but there's all cartridge fuses.

 

[Southcoast2015]

It seems that there is only 1 type of fitting with 1 circuit in it so it runs on AC under normal conditions and DC in a power failure.

Of course yes - The battery is a centralised unit - No local batteries!

 

[HandySparks]

Nope the max PFC for the cct is just after the MCB, to get near 6Ka, you need to dead short the cct about 1mm after the MCB to get almost zero resistance - generally physically impossible

OK, take a real-world example.

230V supply, PFC of 10kA.

Source impedance is 230/10000 = 0.023ohm

To bring the fault current down to 6kA, fault loop impedance needs to be 230/6000 = 0.038ohm

The difference is 15mohm.

If a final circuit is wired in 10mm² cable, with a circuit resistance of 3.66mohm/m

Cable length required to bring the fault current down to 6kA is 15/3.66 = 4.1m

A bit longer than 1mm.