So this weekend just gone, there was an incident when an artist decided that pouring beer into a 4 way on stage was a good idea. Safe to say it wasn’t and it tripped the RCD for stage power. But it also tripped two more upstream RCD’s, taking out monitor world.
This has got me thinking about the best possible way to avoid this in the future.
The first thing to understand is what happened on this gig, why it failed and why it took out all of the stage and mon’s power.
- The main distro there was a 63a 30mA RCD over 2 32A outputs,
- One fed the monitor racks and the other fed a small distro,
- The small distro had 30mA 16a RCBO’s on 4 16A outputs, powering the monitor desk, the stage power, and most importantly the fridge and kettle,
- Stage power had a further distro with one 16a RCD/BO with 4 16A outputs running to various on stage 4ways.
On this gig, the main PA was fed from the other 2 phases and FOH power was ran from a 16a on the distro on a differing RCD. The main RCD on the main distro was of a higher trip rating than 30mA. This was lucky as loosing FOH power or PA power would have been significantly more of an issue.
The key thing here is that the water/beer in the 4way caused a cascade of RCD/BO trips, firstly the stage power distro, then the small distro then the main distro. When powering back up to identify where the fault was (not knowing where at that time) the RCBO’s caused an issue of upstream tripping.
The key thing I’ve learnt from this is that RCBO’s should NOT be used for stage power as isolators to fault find, and as such they should not be used for stage power. It’s a better idea to have RCD’s with separate MCB isolators. This allows you to isolate the circuit before resetting the RCD, then when resetting the MCB, if the earth leak still exists, it’ll trip that RCD and not an upstream one (assume your discrimination between RCD’s is correctly set up).