new minor works certificate for a fused spur

[gmdaly]

Hi, can someone give me some advice RE completing a small works certificate for a fused spur coming from a ring final.

1) In part 2 of the AMD2 small works cert, its wants Zdb supplying the final circuit. Does this mean the Zs for the ring final that I measure or the MaxZs 32A MCB which is 1.1?
2) for part 3 of the cert, is the new FCU 13A then in effect the new DB and therefore I would put in FCU, BS 1362, 13A?
3) Then for part 4 test results do I complete the R1+R2 just for the cable from the new socket to the FCU? will my max Zs be the rating for the 1362 fuse at 1.83?

Thanks for your help,

gordon daly

 

[Julie.]

Still think its bad design, using 1362's to beat disconnection times, and we can't stop people meddling inside a CU, but swopping fuses inside a FCU is pretty simple mistake to make.

But it's not beating anything.

A fuse is a fuse, if you need to disconnect within 5s or 0.4s you choose the cable/ocpd combination to ensure you achieve the disconnection time.

If your radial can't achieve 0.4s with a 20A mcb you would likely replace it with a 16A and so on.

A fuse is no different, sure it was easier to replace a 5A fuse wire with 30A, or slip a 30A in place in the case of cartridge fuse, than it is to replace a mcb. But it didn't stop us designing lighting circuits using cables incapable of providing disconnection times or overload protection if the user changed the fuse size to 30A.

In the case of a likely fused spur there are going to be situations where the end point doesn’t quite make the disconnection time for a 32A mcb (even though the rfc just makes it), but you cannot say that the system as a whole isn't protected properly as the 13A fuse (maximum) in the fcu fills the gap, just as it does for the overload aspect.

If it's acceptable to reduce the cable cross section at the fcu because the existence of the fuse provides the appropriate overload protection, why would you have to size this same cable up to achieve a disconnection time ignoring the existence of the fuse?

 

[Midwest]

But it's not beating anything.

A fuse is a fuse, if you need to disconnect within 5s or 0.4s you choose the cable/ocpd combination to ensure you achieve the disconnection time.

If your radial can't achieve 0.4s with a 20A mcb you would likely replace it with a 16A and so on.

A fuse is no different, sure it was easier to replace a 5A fuse wire with 30A, or slip a 30A in place in the case of cartridge fuse, than it is to replace a mcb. But it didn't stop us designing lighting circuits using cables incapable of providing disconnection times or overload protection if the user changed the fuse size to 30A.

In the case of a likely fused spur there are going to be situations where the end point doesn’t quite make the disconnection time for a 32A mcb (even though the rfc just makes it), but you cannot say that the system as a whole isn't protected properly as the 13A fuse (maximum) in the fcu fills the gap, just as it does for the overload aspect.

If it's acceptable to reduce the cable cross section at the fcu because the existence of the fuse provides the appropriate overload protection, why would you have to size this same cable up to achieve a disconnection time ignoring the existence of the fuse?

You have your view which I respect, but however I still think it’s a poor deign and practice, to achieve disconnection time using an FCU etc, because the additional cable is too long or too small, when other installation methods can be used.

I think the concept of a FCU is for the connection of appliances, much like a plug top, or limitation of current as suggested in Appendix 15.

 

[sheeshaa1]

I'm aware, a FCU is used to limit current to the extended circuit (same as fuse in plug top to flexible cord). A Zs should conducted at furthest point, are compared to the limiting vales for the measured Zs, against the ocpd at the origin of the circuit, NOT a BS1362 fuse, otherwise its a poor design. As said one could extend a circuit, using a 5A 1362 fuse, to assist the limiting values, only for the owner to replace it with a 13A one.

 

[Pretty Mouth]

I'm aware, a FCU is used to limit current to the extended circuit (same as fuse in plug top to flexible cord). A Zs should conducted at furthest point, are compared to the limiting vales for the measured Zs, against the ocpd at the origin of the circuit, NOT a BS1362 fuse, otherwise its a poor design. As said one could extend a circuit, using a 5A 1362 fuse, to assist the limiting values, only for the owner to replace it with a 13A one.

Struggling to understand your way of thinking.

Like any replaceable fuse, there is a risk that it could be replaced with one of an incorrect rating, that is true. This could lead to overload protection of the extended circuit being omitted - a potentially dangerous situation. I assume you consider this risk to be acceptable, and are happy that a BS1362 fuse offers adequate protection against overload? So why not use it for protection against shock?

 

[Midwest]

Struggling to understand your way of thinking.

Like any replaceable fuse, there is a risk that it could be replaced with one of an incorrect rating, that is true. This could lead to overload protection of the extended circuit being omitted - a potentially dangerous situation. I assume you consider this risk to be acceptable, and are happy that a BS1362 fuse offers adequate protection against overload? So why not use it for protection against shock?

You correct to draw an analogy with rewireable fuses and such like. That’s perhaps why they have been made obsolete.

When designing an installation, you have a duty of care to the user. Historically the were just two BS1362 fuse ratings, 3A & 13A, designed to protect the supply leads in plug tops and the like. As previously said, FCU are intended to be used for fixed appliances, and 13A fuse to protect their supply leads. The use of an FCU as shown in Appendix 15, is the only place I’ve read suggesting this design.

Its the use of the fuse to achieve disconnection times I think is wrong. If the additional cable is too small or too long, then a different design should be used, not reliance on a fuse that can inadvertently changed, causing a danger to the end user.

 

[Pretty Mouth]

You correct to draw an analogy with rewireable fuses and such like. That’s perhaps why they have been made obsolete

They haven't been made obsolete. Here's table 41.2, that includes max Zs for both 3036s and 1362s to meet 0.4s disconnection times:

 

[Pretty Mouth]

Its the use of the fuse to achieve disconnection times I think is wrong. If the additional cable is too small or too long, then a different design should be used, not reliance on a fuse that can inadvertently changed, causing a danger to the end user.

Do you feel that it is okay to use these fuses as protection against overload?

 

[Midwest]

Of course they have a table, but are they manufactured anymore? Only because Appendix 15 uses them as such.

My point is, it’s a poor design, to use to achieve disconnection times.

We can go on arguing the point, but guess we should agree to disagree.

 

[Julie.]

Of course they have a table, but are they manufactured anymore? Only because Appendix 15 uses them as such.

My point is, it’s a poor design, to use to achieve disconnection times.

We can go on arguing the point, but guess we should agree to disagree.

I think the problem is it is difficult for us to understand.

Fuses have been and still are a valuable method of protection still used throughout the industry for both overload and fault current protection.

This is especially the case in industry where the higher fault currents and "stacking" of devices make fuses much more suitable than mcbs/mccbs - although this gap is closing.

They are still used as the main protection in the home for the incoming service, and at the other end - fcus, plugs, and internally in many pieces of equipment.

So I do find it difficult to understand your objections in this particular case. (No disrespect of course, just struggling to understand it)

I actually could/do understand the opposite view - certainly I am still somewhat doubtful about the suitability of mcbs rcds etc as these are mechanical devices which can stick, fail to trip etc (and do on occasion) and other than the explosive charge failing at MV , fuses just don't fail to operate.

Even to this day, I feel more confident relying on fuses than mechanical devices for protection.

 

[Pretty Mouth]

Of course they have a table, but are they manufactured anymore? Only because Appendix 15 uses them as such.

My point is, it’s a poor design, to use to achieve disconnection times.

We can go on arguing the point, but guess we should agree to disagree.

I could understand your point of view if you said 'I don't like any user replaceable fuses as replacing them with one of the the incorrect rating would compromise all protection that they are intended to provide'. This would make sense (but would be impossible to work to in practice, as it would prohibit BS1363 plugs and sockets).

What I don't understand is why you find them acceptable for one type of protection (overload), but not another (disconnection times).

 

[Midwest]

I think the problem is it is difficult for us to understand.

Fuses have been and still are a valuable method of protection still used throughout the industry for both overload and fault current protection.

This is especially the case in industry where the higher fault currents and "stacking" of devices make fuses much more suitable than mcbs/mccbs - although this gap is closing.

They are still used as the main protection in the home for the incoming service, and at the other end - fcus, plugs, and internally in many pieces of equipment.

So I do find it difficult to understand your objections in this particular case. (No disrespect of course, just struggling to understand it)

I actually could/do understand the opposite view - certainly I am still somewhat doubtful about the suitability of mcbs rcds etc as these are mechanical devices which can stick, fail to trip etc (and do on occasion) and other than the explosive charge failing at MV , fuses just don't fail to operate.

Even to this day, I feel more confident relying on fuses than mechanical devices for protection.

I think me and other member were discussing rewireable fuses? I’m not very confident in them.

 

[Midwest]

I could understand your point of view if you said 'I don't like any user replaceable fuses as replacing them with one of the the incorrect rating would compromise all protection that they are intended to provide'. This would make sense (but would be impossible to work to in practice, as it would prohibit BS1363 plugs and sockets).

What I don't understand is why you find them acceptable for one type of protection (overload), but not another (disconnection times).

The addition should be designed around the ocpd at the origin. Take your point about overload, but isn’t disconnection about in part safety to individuals?

 

[Julie.]

The addition should be designed around the ocpd at the origin. Take your point about overload, but isn’t disconnection about in part safety to individuals?

I still go back to my points above, why is a fuse downstream of something ok normally, but not in this case of a 13A style fuse.

The ocpd at the origin is really the incoming (80A bs88) fuse, do we need to ensure all circuits out of the cu operate this fuse within the 0.4s?

Or do we ensure that the ocpd for each circuit is the one to operate in 0.4s?

Obviously we prefer that the downstream ocpd operates first to give selectivity, if this fails and the bs88 fuse goes anyway that's great - but this latter bit doesn't have to operate in 0.4s like the circuit ocpd.

A downstream fuse for the circuit ocpd is just the same, I would much prefer the fuse (plug or fcu) to operate ahead of the circuit ocpd, and that to operate ahead of the incoming bs88 fuse.

Providing each ocpd disconnects within a suitable time for the section it protects - to me that is ideal

The 13A fuse ensures 0.4s downstream of it.

The circuit ocpd ensures 0.4s downstream of itself at least to the 13A fuse (further is not always a bonus, because it could hurt selectivity)

The incoming bs88 fuse ensures 5s downstream of itself at least to the circuit ocpds (further is not always a bonus, because it could hurt selectivity)

I just can't see any disadvantages hence why I am struggling with your reasoning, if I understood, it could change my viewpoint.

 

[Midwest]

I’ll try to be more succinct. The 1362 fuse can be changed easily. If a lower fuse is used to achieve disconnection times, and is subsequently changed to a higher fuse, then its a poor design.

As said, I think we should agree to disagree.

 

[gaxxter]

I’ll try to be more succinct. The 1362 fuse can be changed easily. If a lower fuse is used to achieve disconnection times, and is subsequently changed to a higher fuse, then its a poor design.

As said, I think we should agree to disagree.

we put a label on max fuse size blah take pic covered job done?