Dj Dunc Posted October 26, 2015 Share Posted October 26, 2015 Hello all. I've recently started at a Boarding School as a Theatre Technician. In their main theatre it has a fairly convoluted grid made up of scaffolding and extension cables which I am trying to get shot of in favour of IWBs for the lateral runs. However what I can't get my head around is there are two boxes on the wall loaded with RCBOs, with 15A tail plugs going to the betapack dimmers out of the side, and then some form of patch panel on the front. Now I personally have never seen this arrangement of RCBOs inline between the dimmer and the grid, but then again I'm not a very seasoned or well travelled technician just yet. Can anyone fill me in as to any possible reason that this has been done? TIA Duncan Link to comment Share on other sites More sharing options...
kgallen Posted October 26, 2015 Share Posted October 26, 2015 From an over-current perspective this is potentially redundant as the Betapacks will have 10A HRC fuses or MCBs (depending on which Betapack version you have) per channel. Depends on the over-current rating of the RCBO - 10A is available in BSEN61009. Maybe they wanted RCD protection per channel for the outlets? Is the RCD rating 30mA? Could be an electrician that is not overly familiar with theatre lighting thought, "Oh heck, here's some 15A plugs that don't have fuses and a 'box thing' without RCD final circuit protection, better add some over-current protection and RCDs on these circuits"? Overall, yes it' looks a bit "unusual". Is the supply to the Betapack RCD protected at 30mA? Regards,Kevin Link to comment Share on other sites More sharing options...
DrV Posted October 26, 2015 Share Posted October 26, 2015 It does sound a bit unusual, but the obvious advantage is that it should make it less likely that a lantern fault will take out any more than its own circuit. If you think about the alternative, and more common arrangement, where you have an RCD or RCBO feeding a Betapack, then an earth fault on one lantern will take out 6 channels. However, the down side is that it does not provide any protection for a cable plugged directly into the Betapack. Dave Link to comment Share on other sites More sharing options...
itiba Posted October 26, 2015 Share Posted October 26, 2015 BS7909 permits up to 6 circuits to be protected by one RCD so having a betapack (or other 6 chan dimmer) on a 30mA RCD should be fine. Such an arrangement sounds a rather expensive way of providing additional protection however it might be worth checking the RCBO's are of type A or B on the RCD side of things rather than type AC as the latter isn't as good on a dimmed supply. Link to comment Share on other sites More sharing options...
TomHoward Posted October 26, 2015 Share Posted October 26, 2015 Although an unusual arrangement, a blown lamp may be more likely to trip the RCBO than the HRC fuse in the beta pack, which at least means you don't need a supply of unusual fuses any more? Avoids that age old 'kit kat wrapper around a fuse'.Thanking about that - have you checked that the fuses are still present at the output stage of the beta pack - it isn't part of a wider bodge? Link to comment Share on other sites More sharing options...
matt c Posted October 26, 2015 Share Posted October 26, 2015 Thanking about that - have you checked that the fuses are still present at the output stage of the beta pack - it isn't part of a wider bodge? Like a nicely machined piece of brass rod in place of the fuse? ;-) Link to comment Share on other sites More sharing options...
Dj Dunc Posted October 26, 2015 Author Share Posted October 26, 2015 Just checked and the fuses are indeed present on the betapacks. The RCBOs are 30mA and are labelled as "Type 2". I suspect Kevin has hit the nail on the head as to the why. Just have to decide if I upgrade to 24 Ways of dimming permanently, weather to expand the RCBO set up, do half and half or do away with it. Link to comment Share on other sites More sharing options...
kgallen Posted October 26, 2015 Share Posted October 26, 2015 If the Betapack main supply is RCD protected at 30mA, then your RCBO box could probably go (aside from the "patch panel" benefit). There would be no RCD discrimination between the RCBOs and the Betapack supply RCD, so you have no guarantee of "per channel" protection. If there was an earth fault on one channel then it could be the per-channel RCBO that trips, or the main supply to the Betapack(s) which would take all 6 channels, or both. So in that respect they offer no additional fault protection and create more fault control locations to check. If the Betapack supply has no RCD, then your RCBO thingies could stay, maybe, if they are accessible to reset. If the Betapack main supply has a time-delayed 100mA RCD, then the RCBO box could stay as there would be RCD discrimination. For over-current/overload, the fuse will be faster acting than the MCB(*), but there is no tangible discrimination between a 10A MCB and a 10A HRC fuse, so again, maybe more of a pain than a benefit? HTH. Kevin (*) I dont have the HRC fuse vs MCB fault operating times to hand, but as I understand it, fuse-protected Betapacks used 25A triacs and MCB-protected Betapacks use 40A triacs because of the longer trip time of the MCB versus the fuse. Link to comment Share on other sites More sharing options...
alistermorton Posted October 26, 2015 Share Posted October 26, 2015 Like a nicely machined piece of brass rod in place of the fuse? ;-) Cut down earth pins from 15A plugs work well in 13A plugs. So I'm told. Link to comment Share on other sites More sharing options...
Dj Dunc Posted October 26, 2015 Author Share Posted October 26, 2015 Just dug further into the mess that is the power distribution in here. There is no RCBO upstream of the hard power for the dimmers, it looks like it goes straight into an oldschool blade fuse style distro box which is in turn fed from some location within the nearest 5 miles I assume as nothing is labelled up. Time for a call to the site sparky to see if we can have a wee bit of a tidy up around here! Link to comment Share on other sites More sharing options...
itiba Posted October 26, 2015 Share Posted October 26, 2015 The RCBOs are 30mA and are labelled as "Type 2". Well I suspect they are quite old then as modern over current devices have types B, C or D - numerical types are from the days of BS3871. Any chance of a pic or a BS number? Link to comment Share on other sites More sharing options...
Junior8 Posted October 26, 2015 Share Posted October 26, 2015 Just dug further into the mess that is the power distribution in here. There is no RCBO upstream of the hard power for the dimmers, it looks like it goes straight into an oldschool blade fuse style distro box which is in turn fed from some location within the nearest 5 miles I assume as nothing is labelled up. Time for a call to the site sparky to see if we can have a wee bit of a tidy up around here! Even so what you described in the OP seems a very odd way of going about things - was it run up by some staffer rather than a pro install company? If you do expand get the incomer supply done properly once and for all with each pack fed from a protected industrial type skt. But based on bitter experience find out just what the rating of the feed to the dimmers is. If it is a school install it may have been done on the assumption of 500w only lanterns and not many of them. This seems odd these days but say over thirty years ago it was quite common to assume that loads on a school stage wouldn't go much over 30 amps total - probably based on folk memory of the Junior 8! Link to comment Share on other sites More sharing options...
Peter18 Posted October 26, 2015 Share Posted October 26, 2015 Just dug further into the mess that is the power distribution in here. There is no RCBO upstream of the hard power for the dimmers, it looks like it goes straight into an oldschool blade fuse style distro box which is in turn fed from some location within the nearest 5 miles I assume as nothing is labelled up. Time for a call to the site sparky to see if we can have a wee bit of a tidy up around here! I think I am familiar with this theatre (see my PM).The site sparky is probably not the preferred option... Link to comment Share on other sites More sharing options...
richardash1981 Posted October 27, 2015 Share Posted October 27, 2015 Can I ask a question back one step? I thought that most RCBOs were now active (electronics inside), and so will only actually trip if there is voltage between Live and Neutral available to run the electronics. So I would be quite concerned that it might be possible to have a dimmer channel far enough up to give a shock, without there being enough volts for the electronics to successfully trip. I have always assumed that you just can't put an RCD on dimmed circuits for this reason, or rather it has to go before the dimming system (e.g. a row of RCBOs feeding the modules within the dimmer, instead of a fuse group like Strand used to use). This is arguably a bigger risk than them not existing, because people think they are protected but are not. Link to comment Share on other sites More sharing options...
itiba Posted October 27, 2015 Share Posted October 27, 2015 Modern RCD's run on current rather than voltage as it's the current that creates a magnetic field rather than the voltage. Having said that you still need in excess of 50v for the RCD to operate but anything less than that is outside the scope of the wiring regs. You would normally put them behind the dimmer modules as you state but that's because you'd need to have some form of over current protection there to protect the module so might as well put additional protection there too. If you have RCD's on a dimmed supply you'll need either an A type or B type rather than AC types which is common in domestic installations. Link to comment Share on other sites More sharing options...
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