Satisfying over current characteristics of RCBO's

[EDoubleD]

I work in the events industry which works under 7909. This assumes that someone qualified 7671 etc has tested the distor's and designed a system before it is sent to an event where people like me with 7909 then plug in and do some basic testing.

I've completed 7671 and other domestic install courses with a view to becoming fully qualified in time. At the moment I just work at the 7909 level with a pre done design provided by someone else.

Due to the distances we often run in events RCBO's are standard on every circuit. One thing I need clearing up is the requirements to satisfy both fault and over current protection for an RCBO. Max ELI on an RCD is given in table 41.5, so 1667 for a 30ma (obviously no one wants to see anything near that figure).....so no problem so far.

So if we have 100m run on 16a 2.5mm Ho7-RNF we need to be sure we're going to make 0.4/5s ADS. Am I right in the following thinking:

R1+R2/KM for 2.5mm HO7 is 7.41ohm. So for the 100m run it's going to be 0.74 ohms. We then refer to table 41.3 which give max ELI of 1.37ohms for a type c 16a MCB. Zs is likely in excess of 1.37 as we'll be TN-S and many hundreds of M to get our final 16a circuit (hence RCBO's on every circuit). As we're just using the protection for overcurrent and short circuit the same values are applicable to R1 + RN. Assuming that we're under the 1667ohms for earth loop we are then still making the disconnect times for fire and short circuit, PSCC? The RCBO is type c curve and so we'll be seeing in excess of 160a PSCC based on the values give in table 41.3?

Obviously this excludes any voltage drop considerations on the circuit but I want to check my understanding and thought process of reffering to the regs.

Thanks for the help

 

[plugsandsparks]

Interesting issue: There is a guy on this forum whose day job is just this so hopefully will be along later.

The breaker needs to trip on final ccts in 0.4S say, when you look at two fault types, fault L-E and then fault L-N,
L-E can be captured by the RCD element and hits the disconnection time. Faults L-N if these are perfect faults with no leakage to earth, then yes 0.4S could be a challenge.

 

[Lister1987]

I work in the events industry which works under 7909. This assumes that someone qualified 7671 etc has tested the distor's and designed a system before it is sent to an event where people like me with 7909 then plug in and do some basic testing.

I've completed 7671 and other domestic install courses with a view to becoming fully qualified in time. At the moment I just work at the 7909 level with a pre done design provided by someone else.

Due to the distances we often run in events RCBO's are standard on every circuit. One thing I need clearing up is the requirements to satisfy both fault and over current protection for an RCBO. Max ELI on an RCD is given in table 41.5, so 1667 for a 30ma (obviously no one wants to see anything near that figure).....so no problem so far.

So if we have 100m run on 16a 2.5mm Ho7-RNF we need to be sure we're going to make 0.4/5s ADS. Am I right in the following thinking:

R1+R2/KM for 2.5mm HO7 is 7.41ohm. So for the 100m run it's going to be 0.74 ohms. We then refer to table 41.3 which give max ELI of 1.37ohms for a type c 16a MCB. Zs is likely in excess of 1.37 as we'll be TN-S and many hundreds of M to get our final 16a circuit (hence RCBO's on every circuit). As we're just using the protection for overcurrent and short circuit the same values are applicable to R1 + RN. Assuming that we're under the 1667ohms for earth loop we are then still making the disconnect times for fire and short circuit, PSCC? The RCBO is type c curve and so we'll be seeing in excess of 160a PSCC based on the values give in table 41.3?

Obviously this excludes any voltage drop considerations on the circuit but I want to check my understanding and thought process of reffering to the regs.

Thanks for the help

With 7909 thr main concern is short circuit currents and adiabatic calculation; you can easily satisfy ADS with the 1667 ohm limit so the focus needs to be on PSSC limits as you may find a short circuit won't activate the protective device before the cable starts to degrade.

SparkyNinja/ACES do a variety of 7909 courses (A-D and C&G accredited), they did do an introduction to 7909 webinar during lockdown, you may find that helpful with relation to the above.

 

[EDoubleD]

Interesting issue: There is a guy on this forum whose day job is just this so hopefully will be along later.

The breaker needs to trip on final ccts in 0.4S say, when you look at two fault types, fault L-E and then fault L-N,
L-E can be captured by the RCD element and hits the disconnection time. Faults L-N if these are perfect faults with no leakage to earth, then yes 0.4S could be a challenge.

I think the reality is that the most likely cause of a short circuit is going to be mechanical intevention on a cable, very unlikely that a cable would be cut/sliced just through L & N with zero contact to CPC so yes the RCD is likely to go....but what happens if we got a short in a peice of equipment for example a Capacitor blowing? This could create a short, not all events equipment is protected by BS1363 fuses which would see the current first.. Most lighting will be fed by 20a powercon link cables from soca pex looms all with 16a protection.

 

[pc1966]

For a L-E fault the RCD side of a RCBO will always meet the disconnection time for any likely cable length, and probably it is needed for the "additional protection" against shock at some place.

For the L-N fault you might not meet 0.4s, in fact you can think of cases where you would never disconnect on such a fault but still meet the regs!

For example, a very long run of thin cable feeding a small fixed load. For example, 760m of 1mm and a remote LED floodlight. You might meet the 3% VD requirement (6.9V) for, say 0.25A for a 50W light with a cable resistance of 27.6 ohms. But then on a hard fault you would get 8.3A flowing so it might take forever even on a 6A MCB, let alone a 10A MCB that would be safe for 1mm cable, etc.

While technically within the regs, I would not be happy with such a situation as nobody might notice and yet it is a fault and an enormous waste of electrical power (and thus money). There a 3A FCU as a switch might be an option, as it can be hard to find RCBO below 6A.

The regs only demand that you are safe for L-N faults, so you don't overheat the cable either on CCC during normal operations, or under fault where the MCB side is within the cable's safe envelope of long-term CCC and short-term adiabatic for high current faults.

 

[EDoubleD]

For a L-E fault the RCD side of a RCBO will always meet the disconnection time for any likely cable length, and probably it is needed for the "additional protection" against shock at some place.

For the L-N fault you might not meet 0.4s, in fact you can think of cases where you would never disconnect on such a fault but still meet the regs!

For example, a very long run of thin cable feeding a small fixed load. For example, 760m of 1mm and a remote LED floodlight. You might meet the 3% VD requirement (6.9V) for, say 0.25A for a 50W light with a cable resistance of 27.6 ohms. But then on a hard fault you would get 8.3A flowing so it might take forever even on a 6A MCB, let alone a 10A MCB that would be safe for 1mm cable, etc.

While technically within the regs, I would not be happy with such a situation as nobody might notice and yet it is a fault and an enormous waste of electrical power (and thus money). There a 3A FCU as a switch might be an option, as it can be hard to find RCBO below 6A.

The regs only demand that you are safe for L-N faults, so you don't overheat the cable either on CCC during normal operations, or under fault where the MCB side is within the cable's safe envelope of long-term CCC and short-term adiabatic for high current faults.

The reality of events is that fixed loads are quite rare. Mostly lights going on/off, bass amps pulling variable current, large caterers turning equipment on/off means we often see very inductive loads at the generators and balancing can be a nightmare, even with a design the reality of final use often means we're moving circuits around on 3 phase systems to get some balance to avoid large neurtal currents etc.

The other thing we do is jump up cable sizes a lot. So a 150m 16a run might only have 2 or 3 amps of load on it but will be standard 2.5mm or regularly step up to 6mm. or we can put in line boxes with 6 or 10a MCB's but I still need to look at the calcs to work out whether table 41.5 is fine for general use or if I need to be doing the adiabatic equation every time? Just trying to understand if my thinking of using that table is correct?

 

[pc1966]

Table 41.5 is really about the resistance that will ensure the RCD trips before you exceed a touch voltage of 50V. For any cable I can't see it not being met, but for earth electrodes of course it can be a problem for higher current RCDs at generator outputs, etc.

I would have to check but I suspect if the supply MCB/RCBO is rated below the CCC of the cable(s) fed from it then the adiabatic will always be met. Usually you only have to worry for reduced-size CPCs, or for very high fault currents where the I2t let-thtough of MCB and MCCB can be quite high (but I imagine for temporary sites off generators you won't see anything like the PFC that the grid would deliver for a given supply kVA rating).

 

[Rockingit]

Interesting. For the most part, you're over-thinking this. ADS based on PFC / PSCC on average events sized gensets is a Unicorn anyway so largely irrelevant, hence the use of RCD's to provide protection. Direct shorts do happen, I had a 1600A panel go bang on me backstage last week for just this reason (poor internal workmanship) and in that instance it did take out the upstream (non RCD) upstream ACB but that's probably only as the global powerplant was 2.5MVA so an instantaneous fault current of around 16,000A was achievable. An average 100KVA with a Zgen of 0.25Ohms-ish running a stage and a bunch of caterers you'd be lucky to see 3-400A and by the time you did the RCB/O should have tripped anyway. Also, referring back to your OP, I'd respectfully suggest that 100m+ 16A runs is poor design anyway. Yes, it happens, we've all been there, but you're unlikely to meet Vd anyway at that point so again, ADS suffers massively before you've started.

 

[Electro-tech]

I take it that RCBO type c was chosen to allow for switching inrush currents over the distance.?

 

[Rockingit]

I take it that RCBO type c was chosen to allow for switching inrush currents over the distance.?

C is a standard Type in our world for reasons of inrush

 

[EDoubleD]

Interesting. For the most part, you're over-thinking this. ADS based on PFC / PSCC on average events sized gensets is a Unicorn anyway so largely irrelevant, hence the use of RCD's to provide protection. Direct shorts do happen, I had a 1600A panel go bang on me backstage last week for just this reason (poor internal workmanship) and in that instance it did take out the upstream (non RCD) upstream ACB but that's probably only as the global powerplant was 2.5MVA so an instantaneous fault current of around 16,000A was achievable. An average 100KVA with a Zgen of 0.25Ohms-ish running a stage and a bunch of caterers you'd be lucky to see 3-400A and by the time you did the RCB/O should have tripped anyway. Also, referring back to your OP, I'd respectfully suggest that 100m+ 16A runs is poor design anyway. Yes, it happens, we've all been there, but you're unlikely to meet Vd anyway at that point so again, ADS suffers massively before you've started.

Thanks for the reply. yes probably over thinking it and like you say ADS on PFC almost unheard of which is why any decent distro will have at least a global rcd but ideally rcbos (for reasons we both understand ). Saying that I have seen essential supplies made 32a PCE handbags with 4 x MCB and no RCD....not ideal depending on what they're plugged into.

Anyway...yes 100m 16a would be a bad design it was just an example to illustrate what I was trying to get at.....but real world example that got me really thinking about it.

Glastonbury this year we're plugging in a load of bell tents for one of the landowners around gate c area. He's had an 11kv transformer installed and a TT supply. We're plugging in runs of 12 bell tents at a time for mobile phone charging. The plan we were provided showed bell tents at 2m spacing. In the middle of each run is a 32a post (so 32a per 24 tents). We're running 13a 2 ways fused at 3a on a 16a t split on 2.5mm. Each tent was then allocated 5m of cable so the design calls for 60m of cable per run, no problem here. However, on arriving to site it's clear that due to a variety of reasons including the tents being further spaced 10m cable is required. We're there ready to go but basically without the bits we really need to make it all work and as you know there's often no coming back tomorrow at a show. At 120m on 2.5mm we're getting (from memory) somewhere in the region of 2.4ohms and 89a PSCC. Not good on a 16a C curve RCBO. In this instance we went down to screwfix in shepton and made some single gang 6a C curve I/O boxes for further along the runs to provide protection where the figures weren't great. The 13a outlets are fused at 3a as it's really just for phones but of course somone may decide to plug in a hairdryer etc.

General feeling was we'd make VD as the load was so minimal at any one time. Maybe 24 mobiles per line overnight, max 1 or 2 amps.

The whole thing got me thinking about PSCC and the calcs needed before we get to site. In this scenario (excluding the changes we made on site) would you be using the table I've highlighted or the adiabatic equation during the design phase or am I literally over thinking the whole thing?

 

[Rockingit]

Thanks for the reply. yes probably over thinking it and like you say ADS on PFC almost unheard of which is why any decent distro will have at least a global rcd but ideally rcbos (for reasons we both understand ). Saying that I have seen essential supplies made 32a PCE handbags with 4 x MCB and no RCD....not ideal depending on what they're plugged into.

Anyway...yes 100m 16a would be a bad design it was just an example to illustrate what I was trying to get at.....but real world example that got me really thinking about it.

Glastonbury this year we're plugging in a load of bell tents for one of the landowners around gate c area. He's had an 11kv transformer installed and a TT supply. We're plugging in runs of 12 bell tents at a time for mobile phone charging. The plan we were provided showed bell tents at 2m spacing. In the middle of each run is a 32a post (so 32a per 24 tents). We're running 13a 2 ways fused at 3a on a 16a t split on 2.5mm. Each tent was then allocated 5m of cable so the design calls for 60m of cable per run, no problem here. However, on arriving to site it's clear that due to a variety of reasons including the tents being further spaced 10m cable is required. We're there ready to go but basically without the bits we really need to make it all work and as you know there's often no coming back tomorrow at a show. At 120m on 2.5mm we're getting (from memory) somewhere in the region of 2.4ohms and 89a PSCC. Not good on a 16a C curve RCBO. In this instance we went down to screwfix in shepton and made some single gang 6a C curve I/O boxes for further along the runs to provide protection where the figures weren't great. The 13a outlets are fused at 3a as it's really just for phones but of course somone may decide to plug in a hairdryer etc.

General feeling was we'd make VD as the load was so minimal at any one time. Maybe 24 mobiles per line overnight, max 1 or 2 amps.

The whole thing got me thinking about PSCC and the calcs needed before we get to site. In this scenario (excluding the changes we made on site) would you be using the table I've highlighted or the adiabatic equation during the design phase or am I literally over thinking the whole thing?

Purely TT supply is unusal for that but not unheard of. However, the answer is in your question as even though 7909 is in play, 7671 is still the basis and as such for a TT requires RCD's, hence you have the magic permissible 1667Ohms for a Zs.

 

[Rockingit]

Thanks for the reply. yes probably over thinking it and like you say ADS on PFC almost unheard of which is why any decent distro will have at least a global rcd but ideally rcbos (for reasons we both understand ). Saying that I have seen essential supplies made 32a PCE handbags with 4 x MCB and no RCD....not ideal depending on what they're plugged into.

Anyway...yes 100m 16a would be a bad design it was just an example to illustrate what I was trying to get at.....but real world example that got me really thinking about it.

Glastonbury this year we're plugging in a load of bell tents for one of the landowners around gate c area. He's had an 11kv transformer installed and a TT supply. We're plugging in runs of 12 bell tents at a time for mobile phone charging. The plan we were provided showed bell tents at 2m spacing. In the middle of each run is a 32a post (so 32a per 24 tents). We're running 13a 2 ways fused at 3a on a 16a t split on 2.5mm. Each tent was then allocated 5m of cable so the design calls for 60m of cable per run, no problem here. However, on arriving to site it's clear that due to a variety of reasons including the tents being further spaced 10m cable is required. We're there ready to go but basically without the bits we really need to make it all work and as you know there's often no coming back tomorrow at a show. At 120m on 2.5mm we're getting (from memory) somewhere in the region of 2.4ohms and 89a PSCC. Not good on a 16a C curve RCBO. In this instance we went down to screwfix in shepton and made some single gang 6a C curve I/O boxes for further along the runs to provide protection where the figures weren't great. The 13a outlets are fused at 3a as it's really just for phones but of course somone may decide to plug in a hairdryer etc.

General feeling was we'd make VD as the load was so minimal at any one time. Maybe 24 mobiles per line overnight, max 1 or 2 amps.

The whole thing got me thinking about PSCC and the calcs needed before we get to site. In this scenario (excluding the changes we made on site) would you be using the table I've highlighted or the adiabatic equation during the design phase or am I literally over thinking the whole thing?

BTW..... DM me. You can send me your CV.