Dc Through Ac Mcb's

[pickleye]

Hi all,

I'm stuck with remedial a on a large commercial building. PFC is generally 6-10kA and the testers have flagged that most of the breakers are only rated at 6kA, fair cop.

Each of the boards is fed via a BS 88 fuse, typically up to 100amp. The decision has been made (above my head) to change all the breakers to 10kA equivalent.

I'm not long out of my time and this is my first set of remedials. Question, would the fuse provide additional breaking capacity for the board, effectively removing the issue. I've heard of energy let through, but "I don't know the score". Would every board need assessment and data from the manufacturer.

Also, it's a 1960's building with centrally controlled corridor lighting. The feed to board via contactors. But.... If the main lights are off, a number of fittings are left on (AC) for security lighting, ok so far.

In the event of power loss the central battery system fires 270vDC through some of the AC breakers. This hasn't been noted by the testers.

So... The AC PFC exceeds the breaking capacity, but, AC breakers are rated way lower for DC fault current. But the output of the central battery system will be fused.

Do I, well those that decide, need to worry. Again I don't make the decisions but am keen to understand the issues and possible solutions as the powers that be are, erm well erm ... Not deciding very well.

Current favourite is fitting fuses instead of MCB's.

Do l need DC test gear to measure DC fault current? Currently using Metrel, but don't know the model off the top of my head.

Thanks in advance.

 

[Essex1]

If the PFC tested at the board is higher than the rated MCB then they need changing. The BS88 is acted at 80ka so no issue there.

I am interested in other members answers as I am unsure but if it is a UPS that is feeding the emergency lighting then isn't this from batteries? Would there be any fault current from batteries?

 

[pickleye]

It's a central battery system, so yes batteries.

If there was a short after the board, there would be an overload situation that the breaker would need to clear. I don't know what the fault current would be to be honest.

It's new to me, and no one wants to offer a solution or get involved to be honest.

 

[pickleye]

It won't be designed correctly I'm sure. The 6kA's were put in years ago to replace fuses. I've never seen so many c curve MCB's in my life.

The battery system is definitely sending DC out. It caught my volt stick loving mate out ; -)

I've measured it at 274VDC. Theyre in the process of upgrading to led everything and had to wait for the fittings with DC control gear. So yes definitely DC.

 

[ProDave]

I though ac circuit breakers relied on the zero crossing to extinguish an arc.

the same device on dc, may not function correctly.

Or am I talking carp?

 

[pickleye]

That's what I've read Dave. The 0v crossing helps reduce the magnitude of fault to break.

DC Does not do this hence a lot lower rated

 

[pickleye]

I'd love too Canoey.

I believe it was replaced a few years ago. Also they've just shelled out for DC led fittings so it's not an option.

It's not my decision either. I just raised the concern.

Site spark hasn't twigged the problem, testers never noted it either. I just don't want to be the last man on when it goes wrong.

My manager is agency, and the higher ups treat it like a hot potatoe. But it's got to be done.

 

[Essex1]

Is it AC during normal conditions and DC during emergency conditions?

 

[pickleye]

They all sound like muppets......

Never a truer word spoken

Yes AC normal, DC emergency.

 

[Essex1]

So each fitting is going to have a set of AC LEDs and DC LEDs.

 

[pickleye]

Erm... I'd guess so, but I've had nothing to do with them.

The site spark is working his way through 4-500 fittings. He has said in the past there are two circuits in some fittings.

 

[Lurch]

So each fitting is going to have a set of AC LEDs and DC LEDs.

Not necessarily, you can feed a rectifier with AC or DC and DC will come out of the other side.

 

[roys]

What a strange way of doing things, just read it again and I am still can not think why the installation would be designed this way.

Why aren't the batteries just part of a UPS system so that every thing downstream is exactly the same.

 

[pickleye]

I have no idea Roy. I guess it was designed with fuses 40 odd years ago. The fuses were converted to MCB's at some point. I don't think anyone realised the problem.

I guess gear has just been replaced as needed over that time.

A bit of foresight could have saved a lot of pain now.

Straw pole:

Is this an issue or am I being a noobe drama queen?

 

[apprentice87]

I think you are being perfectly sensible. I know absolutely nothing about the systems you describe, but can tell you that when it comes to breaking an arc, DC is vastly different.. Get the fuses back i say!!

john..

 

[steptoe]

Holy **** 

Never heard the like of it

 

[Sidewinder]

Pickle,

You are NOT being a drama queen.

There are serious issues here tgat have not been considered by the designers.

First thing, the configuration of the dc supply must be established.

Shock protection will be required, thus the -ve of the dc will almost certainly have to be earthed.

Then you will need to break the +ve in the event of a fault.

Two pages from an old Merlin Gerin circuit breaker applicationn guide attached.

Batteries have crazy pssc, as they have very low internal resistance and being dc threr is no impedance or reactance etc to oppose the rising fault currents.

To break 50kA not unheard of with a battery system, you would need 4 poles in series for a C60 series (I don't mean a C curve 60A, but the MG product range).

I doubt this is achievable with a conventional A or B type DB.

 

[ProDave]

Apart from the circuit breakers, are all the light switched rated for dc operation?

 

[Essex1]

I don't think they will use the light switches when it goes into emergency mode.

 

[Sidewinder]

PD,

I don't see how the light switches come into it.

The batteries are for emergency lighting, so, it must not be possible to turn off the lights when under emergency conditions.

The test function must be integral with the system, or dc rated, yes.

I am a little more concerned about running the emergcpenc back upmsystem through the duty circuits.

Also, to provide earth fault protection I can't see anyway of achieving this other than an N-E link @ each DB.

The +ve will be down the line,mthe -ve down the N, thus the N& E have to ne linked to provide earth fault protection.

Pickle, I've been driving to the job for an hour, thinking about this.

Are you sure the standby supplies are going through the duty breakers?