RCBOs on the grid feeds.

[Jivemaster]

Determine what you have before you start work. There is little point in upgrading anything if the power needed doesn't come into the building or stage area. Yes I can remember when lamp choice was 250, 300 or 500watts (and the lanterns let little of that actually out the front!).

 

Actually I've heard bad things about RCDs on dimmed/chopped mains Maybe you could confirm that your RCBOs are OK on chopped mains.

[J Pearce]

While voltage before the triac dimming circuit might be higher the current will remain the same (Kirchhoffs current law), so if the RCBO is current operated then there is no difference whether it is before or after the triac dimming circuit.

[richardash1981]

Modern RCD's run on current rather than voltage as it's the current that creates a magnetic field rather than the voltage.

I realise that the sensing is of the differential current. My point was the one that was in pieces on my desk recently fed the signal from the differential transformer which sensed the current into an electronic amplifier, before using a solenoid to trip the mechanism. The amplifier and solenoid were (I think) run from the live-neutral voltage via a series capacitor etc. If that amplifier isn't powered up or there isn't enough power to operate the solenoid, then the RCD can't trip no matter how much differential current there is flowing.

 

This is supposed to be why most plug-in type RCDs are now non-latching and trip to off as soon as the input power is removed, requiring a manual reset before use. That way, if the incomming neutral isn't connected, you can't get a shock from the outgoing live, because it won't reset. It's no good on a dimmer circuit though!

[dbuckley]

I have no measurements or practice to contribute as to the practice of running RCDs or RCBOs from dimmed sources, but in the USA, one can buy a GFCI (their RCD equivalnt) which has separate terminals for continuous supply, as well as in and out for the load, so the electronics are operative irrespective of the supply to the load. The spec sheet for the Leviton Panel-Mount GFCI for Theater Dimming Systems can be downloaded from here.

[Tomo]

Most commonly available miniature RCD and RCBO devices do not offer useful residual-current protection in this situation.

 

It is extremely unlikely that they are rated for use on the output of a dimmer.

If they aren't specifically designed for that use then they are worse than useless as they imply a protection that isn't present.

 

I would recommend that they be removed, and replaced by an appropriate RCD solution at the supply side of each dimmer pack.

- 3-phase TPN RCDs are quite cheap. It is usual to fit one TPN RCD per pack.

 

Quick summary:

Type "A", "F" and "B" RCDs are generally suitable for use on the supply-side of dimmers.

They can detect and open the types of leakage current that occur with dimmers.

 

Type "A" is the most common in theatres. (F and B are special-purpose)

 

Almost all type "A", "F" and "B" RCDs are Voltage-Dependent

This means they use power from the supply they protect to drive their internal electronics and to either keep the contacts closed, or to force them open.

When that is below a certain voltage*, or interrupted (as a dimmer does), they may not operate.

- They either drop open or will never open.

 

Type "AC" RCDs are unsuitable for use with dimmers,and should never be used on the supply or output of them.

 

The common Voltage-Independent RCDs are type "AC" RCDs - which means they're not suitable for use with dimmers!

 

It is possible to buy A, F and B RCDs that are rated for use on the output of a a dimmer, however they are rare and quite expensive.

Many use an external power supply to operate the physical contacts.

 

There is a reason why every dimmer pack you can buy with factory-fitted RCDs puts them on the supply side!

 

* The standard says 85% of nominal, 195V. Some are rated for lower voltages of course.