MCCB and 24v em lighting

gamblor6 · Jan 19, 2012

Hi
2 questions really,
im doing a periodic on an old residential home, one of the mccb,s (an old dorman smith single phase 32a) has a small dial bit with 100%/70% on it. what is this?
also i have a em lighting supply leaving the db going to an isolator then on to a big battery(more like 2 LARGE car batteries connected in series) then out the battery to a single phase db marked 24v em lighting. the mcbs are just normal 240v(as it was in those days) breakers. is this ok on a 24v supply? will 240v mcb,s trip if there is a fault? i dont think this is right but someone may know better
Thanks in advance!!!

johnboy6083 · Jan 19, 2012

the MCCB dial is to set the rating of the device compasred to its nominal rating. So say you have a 100A device, and its set to 70%, its set at 70A.

as for the 24V batteries, i think you need protection for the 24V circuits. I reckon the impedance of the batteries will be far to high to allow disconnection of the circuits by an MCB. A fuse would be good for this, but installed on the 24V side.

gamblor6 · Jan 19, 2012

the mcbs are on the 24v side, just doesnt make sense to me

johnboy6083 · Jan 19, 2012

then to be honest i dont know if they would work on 24v. any 24v stuff i install is PLC/control related, and we use small HRC fuses.

spark 68 · Jan 19, 2012

We have used MCB's in ELV circuits before on control and 24V DC dist. circuits.

johnboy6083 · Jan 19, 2012

apparently schnieder MCB's are rated down to 12V. read this http://www.global-download.schneide...577BC/$File/multi_9_mcb_application_guide.pdf

i would check with the appriopriate manufacturer.
so what the previous installed has done is ok, providing the MCBs are rated down to 12v

spark 68 · Jan 19, 2012

We used ABB ones, As JB said, check with the manufacturer

gamblor6 · Jan 19, 2012

cool cheers guys. they are old dorman smith ones ill give it a look

Knobhead · Jan 20, 2012

Thermal and instantaneous O/L’s in most MCB’s will work. The problem comes when you try to break 230V DC with a 230V AC breaker. The fault will just carry on across the contacts of the MCB.
To give you an idea of this, I pulled a fuse on an old DC system. The arc carried on carrying current across the fuse bridge.
Check with the manufacturers about DC applications.
To be honest the old Dorman Smith units I wouldn’t trust them as far as I could throw them!