UK Selectivity, mcb vs fuse...

[TheArcMan]

Hi all,

Been browsing these forums for a while, always great to learn a new way to skin the same cat.
Anyway, cut a long story short, was an apprentice for a top spark, he packed it in for a new 7 figure income career. I then took a break thinking I should do the same, then I realised not everyone has what it takes to be in the big earner society, so I'm now back, and working for a much older spark.

Thing is, my new boss works with live wires like its perfectly fine, sure he's all PP'd up, but my last boss was ---- about turning everything off, would even walk away from jobs if he couldn't cut-off. I'm in the process of deciding if I should stay with him or move on, or maybe I just need to grow some more chest hair, but thats another story. I guess this is just an age thing, current boss is from a different era.

Now below follows what I've noticed he has done on 2 jobs now and I never seen it done this way before and it has been driving my mind in circles so thought I'd ask and see what others think >

TT supply... Distribution board feeds subs, ie. db is outside in a remote location next to the supply and swa's from db feed all subs (house, garage, outbuilding, guest house, ev charge points etc)

At the db, on outgoing circuits he's using 300ma type-a RCCBs followed by 22x58 fuses on the line, then swa out to sub, where needed he drops type-b 300ma for ev chargers (a pretty penny!). Now he also has a habit of double boxing, ie terminates cw glands on ip68 aluminium boxes inside abs/pc boxes with conduits running into sections of square pvc pipe plumbed into the plastics boxes then all sealed up etc., definitely way more overkill compared to what I've been used to.

What gets me is he says he uses fuses for selectivity, and not mcbs because they'll potentially trip and shut off power to everything at the subs, however I thought you are supposed to use MCBs with RCCBs, I've never seen a fuse being used in front of one before, what am I missing? and wouldn't the fuse blow? which then means square one again but an even bigger hassle for the client? though he says they don't blow that easy with a good mcb doing its job downstream?

 

[MDJ]

Hi all,

Been browsing these forums for a while, always great to learn a new way to skin the same cat.
Anyway, cut a long story short, was an apprentice for a top spark, he packed it in for a new 7 figure income career. I then took a break thinking I should do the same, then I realised not everyone has what it takes to be in the big earner society, so I'm now back, and working for a much older spark.

Thing is, my new boss works with live wires like its perfectly fine, sure he's all PP'd up, but my last boss was ---- about turning everything off, would even walk away from jobs if he couldn't cut-off. I'm in the process of deciding if I should stay with him or move on, or maybe I just need to grow some more chest hair, but thats another story. I guess this is just an age thing, current boss is from a different era.

Now below follows what I've noticed he has done on 2 jobs now and I never seen it done this way before and it has been driving my mind in circles so thought I'd ask and see what others think >

TT supply... Distribution board feeds subs, ie. db is outside in a remote location next to the supply and swa's from db feed all subs (house, garage, outbuilding, guest house, ev charge points etc)

At the db, on outgoing circuits he's using 300ma type-a RCCBs followed by 22x58 fuses on the line, then swa out to sub, where needed he drops type-b 300ma for ev chargers (a pretty penny!). Now he also has a habit of double boxing, ie terminates cw glands on ip68 aluminium boxes inside abs/pc boxes with conduits running into sections of square pvc pipe plumbed into the plastics boxes then all sealed up etc., definitely way more overkill compared to what I've been used to.

What gets me is he says he uses fuses for selectivity, and not mcbs because they'll potentially trip and shut off power to everything at the subs, however I thought you are supposed to use MCBs with RCCBs, I've never seen a fuse being used in front of one before, what am I missing? and wouldn't the fuse blow? which then means square one again but an even bigger hassle for the client? though he says they don't blow that easy with a good mcb doing its job downstream?

welcome aboard, what's your question? also with total respect to you, you may get more replies if your first post in the thread is a bit smaller, most readers will get bored half way through and just not bother reading it all. Great to see you and I hope you enjoy the forum

 

[TheArcMan]

Sorry about the length of the first post.

It isn't so much as a question as it is more a scoping others thoughts on using fuses to protect not only circuits but also RCCBs.

I was under the impression that manufacturers do not advise reliance upon fuses to protect their RCCBs, and that MCBs should be used because of their trip speed. Yes a fuse is well within regulation, however the potential of damaging costly RCCBs is also there?

I guess selectivity meets regs, but on severe faults, customers end up paying more for new RCCBs potentially?

Problem is I can't seem to find any info from any supplier regarding preference between using MCB or fuse for sc/ol faults post RCCB.

 

[pc1966]

When you have a MCB feeding another MCB (or RCBO) then you will get poor selectivity as any fault that hits the "instant" magnetic trip point of the upstream MCB is likely to fire both.

A fuse of suitably rating offers better selectivity as (simplified) the time it takes to heat the wire give the downstream MCB time to clear the fault. Of course, if the fault is big enough both go and then you are looking in to the cascading rating which, for example, allows a 6kA break limit MCB to be safe to 16kA PFC on a domestic CU due to the coordination with the standard 60-100A BS88 fuses used.

If you want good selectivity and a simple means of resetting the upstream protection, so not having to call a spark to change a bolted fuse, etc, then you need to look at a MCCB that has a short-time delay to give you the same sort of advantage. However, they are way more expensive than a fuse, and generally offer less energy-limiting so downstream problems can be bigger.

The coordination of RCCB and OCPD is another example of cascaded ratings, many RCCB can only break 1-1.5kA faults, however, your typical MCB and often the use of a fuse means for faults above 1kA or so they will break before the RCCB starts opening its contacts, allowing a safe interruption of it.

If you want to put some numbers to these ideas, get the Hager commercial distribution catalogue (its about 15MB of a download) and look at the tables near the back, around page 142 you start to get values for selectivity with MCB & BS88 fuses, etc. They show "T" for total selectivity, basically it is always the downstream MCB that goes up to its breaking limit, but more generally they give you the fault in kA that no longer achieves selectivity.

 

[pc1966]

For example:

• 32A B-curve MCB from 63A BS88 fuse = 1.3kA, 5s Zs = 0.62 ohm
• 32A B-curve MCB from 63A D-curve MCB = 1.67kA, 5s Zs = 0.28 ohm
• 32A B-curve MCB from 63A C-curve MCB = 1.11kA, 5s Zs = 0.27 ohm
• 32A B-curve MCB from 63A B-curve MCB = 0.57kA, 5s Zs = 0.56 ohm
• 32A B-curve MCB from 63A x160 series MCCB = 5.17kA, 5s Zs = 0.28 ohm

In this cheap cases the D-curve MCB wins, but then you look at the Zs limits (here from OSG) and you see that the fuse allows a higher Zs than any of the MCBs so is also easier for sub-main design.

Edit 1: Added example MCCB for comparison: best selectivity, comparable Zs to D-curve, highest cost.
Edit 2: Also note these are specific to the Hager NBN/NCN/NDN, etc, series of MCB. Check others as needed if it matters!

 

[TheArcMan]

I was aware of Zs being higher with fuse, the bit I didn't clock onto is the fault limit of RCCBs.

 

[nicebutdim]

I was aware of Zs being higher with fuse, the bit I didn't clock onto is the fault limit of RCCBs.

The guy you're working with sounds like someome a lot could be learnt from and I like the description you've provided of how thorough his work is.

 

[TheArcMan]

I am most definitely aware of that, the only problem is that he doesn't talk much and doesn't like being talked to whilst working so I have to remember what I need to ask after any particular milestone, and by then I'm focused on something else.

On another note, with a little google-foo, I did come across some tech-specs for Doepke's Type-B 10ka RCCBs and lo and behold in there it does state a recommended protection fuse type of 'gG'. So at least that manufacturer confirms fuse compatibility.

 

[pc1966]

For the incomer RCCBs they are usually selective types, so they take about 200ms or so to open to allow downstream RCCB/RCBO time to open first.

If you look at typical time-current plots for fuses like Fig 3A1 in the regs you see that the usual 63-100A fuses are opening in 100ms for currents in the 0.82-1.45kA region, so the RCCB should never have to interrupt more than that as the fuse will have opened (or be current-limiting) before it.

 

[littlespark]

The guy you're working with sounds like someome a lot could be learnt from and I like the description you've provided of how thorough his work is.

Except the bit in the OP where he says

Thing is, my new boss works with live wires like its perfectly fine,

Does that mean he will deliberately do stuff live... just because he cant be bothered to isolate, test, etc etc.....

Does he have as little thought to people he's working with than he does to himself? Just because he thinks the odd belt wont do him any harm...

Apologies if i got the wrong idea there... but it just strikes me as reckless

 

[pc1966]

Except the bit in the OP where he says

He goers on to say with PPE suggesting his boss is not an idiot but has sufficient respect for live systems to wear gloves, arc-flash face shield, etc. But you are right, there is more to the statement that we don't have.

After all there are differing degrees of "live work" and various people and/or businesses draw the line at different positions:

• There is live work in the form of testing an energised system. For example Zs and verification of polarity which we are expected to do, but some companies forbid even this (which begs the question of how reasonable an EICR they could do, etc).
• There is live work in the form of modifying an isolated circuit in close proximity to other energised circuits. For example, altering an existing MCB/RCBO circuit from the point of isolation within an energised DB. Generally nobody would do that in a domestic situation, but in an industrial situation with a lot of always-on critical systems it might be commonly done if the risks are manageable.
• Finally there is LIVE WORK in the form of modifying an energised system. Normally nobody other than specifically trained linesmen for a DNO would attempt this, and only if it was thought to be the only reasonable step under unusual circumstances, but some folks do things like insert a new MCB/RCBO on to an energised DB busbar which is pushing the definition of acceptable safety given the risks of it going in wrong and/or related tools making contact to said busbar when covers are off it, etc.

It comes down to risks and implications for the business. For example, I will do the live work example on my own DB as I know they are safe enough to removed the cover without an unexpected disaster, but for any unknown DB I would be VERY reluctant to take the cover off while energised for the first time!

 

[davesparks]

It isn't so much as a question as it is more a scoping others thoughts on using fuses to protect not only circuits but also RCCBs.

Nothing wrong with it.

I was under the impression that manufacturers do not advise reliance upon fuses to protect their RCCBs, and that MCBs should be used because of their trip speed.

Which manufacturers have you got this information from?
What do you mean by trip speed? Fuses and MCBs can operate in the same time.

Yes a fuse is well within regulation, however the potential of damaging costly RCCBs is also there?

If it is designed correctly with correctly rated RCCBs then they won't be damaged.