Breaking capacity

[Martin Flint]

Hi can anyone advise me. My company have installed a dimming rack ( from USA ) it's a 0-10 v dmx system . It's 3- phase and protected by 3- phase B 20 amp 60898 but on the dimmer rack it has 24 x C10 amp 60898 but they only have a breaking capacity of 3 KA my Pfc reading is 2.9 so it exceeds the B/C as the main 60898 has B/C of 10 K/A is this covered I know under fault condition the dimmers can melt but it will trip through main mcb . Where does that leave me with certification ? Can anyone help please

 

[Rockingit]

It's not a fixed part of the installation, it's an appliance. However, I would look at uprating that B20.....what's the dimmer rack driving?

 

[Martin Flint]

Hi thanks for your reply it's a 3- phase 20 A but it's only doing led lights. So regarding certification the 3- phase mcb will meet requirements even though each led circuit goes straight back to the dimmer rack and there individual mcb only have a B/C of 3K/A and my Pfc is 2.9 and if I'm right in thinking you have to double this reading as it's 3- phase so would exceed the rating on the dimmer panels mcb s

 

[telectrix]

where are you measuring that 2.9kA? before the 3kA breakers or after?

 

[Rockingit]

Its single ph circuits, each to an MCB which the manufacturer has built and specified as they wish - its not your responsibility. You only have to answer for the upstream breaker, and as I said earlier, you may have some issues with 8 x C10's feeding from a B20 due to inrush current, so look to upgrade that breaker to at least a C32/3. The internals of the dimmer handle everything else and ffs don't megger it!!

 

[Martin Flint]

Yeh on the mains before the mcbs

 

[Rockingit]

Out of interest, which brand of dimmer is it?

 

[Martin Flint]

It's a unison DRd6 electronic theatre controls Middleton wi USA

 

[telectrix]

Yeh on the mains before the mcbs

those MCBs only need to be rated for the kA value after the MCBs.

 

[Rockingit]

It's a unison DRd6 electronic theatre controls Middleton wi USA

LOL - AKA an ETC theatre rack. What's the application? Odd choice of weapon for a standard architectural install.

 

[Rockingit]

those MCBs only need to be rated for the kA value after the MCBs.

Tel - doesn't quite work that way. The individual channel MCB's exist solely to protect the channel Triac inside the dimmer / functional isolation, and the pfc of the individual circuit will not resemble the pfc of the incoming arrangement. In fact, you can't (really) in any meaningful way test for the pfc of the outgoing circuits as they've become chopped electronic waveforms via a bunch of timers and triacs. So long as the upstream CPD is rated properly for the right level of 3ph PFC then all is well. What happens after it enters the box is all smoke and mirrors!

 

[telectrix]

i stand corrected. no knowledge of these dimmer arrangements, so will bow to your suprior knowledge on this.

 

[davesparks]

Tel - doesn't quite work that way. The individual channel MCB's exist solely to protect the channel Triac inside the dimmer / functional isolation, and the pfc of the individual circuit will not resemble the pfc of the incoming arrangement. In fact, you can't (really) in any meaningful way test for the pfc of the outgoing circuits as they've become chopped electronic waveforms via a bunch of timers and triacs. So long as the upstream CPD is rated properly for the right level of 3ph PFC then all is well. What happens after it enters the box is all smoke and mirrors!

Whilst my learned friend is quite right about the Pfc after the dimmer, I'd point out that the outgoing wiring from it still needs to be tested properly, just in case his comments are misunderstood.

Personally I treat each dimmer as if it was a DB and each one gets a test results sheet. You can easily do all of the tests without causing any trouble to the electronics, just have to use the alternative method of Zs testing where you link L and E at the far end of the circuit.