table 41.1 max. disconn. times and the extra notes

Glad to be of help -  and do not worry,  I was only trying to warn that this might not actually help either - after all I had understood my own earlier post as well ! but it is hard to guess how others are looking at the same problem.

There is a large random element about the exact time limits for a lot of this - it is partly because we kind of like a rule of 3 for discrimination between successive layers of protection or perhaps 2 and bit if we are in a really tight spot.

So in the dim and distant past this was decided that  the front line may be a 13A fuse, backed by a 30 backed by 60, 80  or 100 and a  few hundred at the substation.

When trips  with both coils and bimetallic strips came along we had the option of a fast and slow part, and  got a bit more scientific with the time to break curves  - the bimetallic part more or less matching the fuse curves of old, handling overloads and the near-instant magnetic trip for dead shorts reducing the stress and permitting smaller contacts and thinner metals in a few places.


The only times that are not simply set by a committee furtively chewing their metaphorical pencils, are the ones set by the human heart beat period and the currents that can cause fibrillation - namely the shock protection table we have been discussing. The slower times are all about trying to avoid unnecessary disconnection of perfectly satisfactory bits of the installation, and at the same time not have too long a fault duration to cause damage, either by overheat or by voltage rises or depressions on the rest of the network. They could probably just  as well have said 4.1 seconds and 1.83 second,but round numbers are easier to use.

There is a lot of the spirit of "measure with micrometer, mark in chalk, cut with axe"  when folk agonise about small changes to Zs or breaking times.  Being just a marginal pass or just a marginal fail is neither that useful or that serious - being a clear pass is what is needed, but that is not compatible with a simple set of instructions.

Glad to be of help -  and do not worry,  I was only trying to warn that this might not actually help either - after all I had understood my own earlier post as well ! but it is hard to guess how others are looking at the same problem.

There is a large random element about the exact time limits for a lot of this - it is partly because we kind of like a rule of 3 for discrimination between successive layers of protection or perhaps 2 and bit if we are in a really tight spot.

So in the dim and distant past this was decided that  the front line may be a 13A fuse, backed by a 30 backed by 60, 80  or 100 and a  few hundred at the substation.

When trips  with both coils and bimetallic strips came along we had the option of a fast and slow part, and  got a bit more scientific with the time to break curves  - the bimetallic part more or less matching the fuse curves of old, handling overloads and the near-instant magnetic trip for dead shorts reducing the stress and permitting smaller contacts and thinner metals in a few places.


The only times that are not simply set by a committee furtively chewing their metaphorical pencils, are the ones set by the human heart beat period and the currents that can cause fibrillation - namely the shock protection table we have been discussing. The slower times are all about trying to avoid unnecessary disconnection of perfectly satisfactory bits of the installation, and at the same time not have too long a fault duration to cause damage, either by overheat or by voltage rises or depressions on the rest of the network. They could probably just  as well have said 4.1 seconds and 1.83 second,but round numbers are easier to use.

There is a lot of the spirit of "measure with micrometer, mark in chalk, cut with axe"  when folk agonise about small changes to Zs or breaking times.  Being just a marginal pass or just a marginal fail is neither that useful or that serious - being a clear pass is what is needed, but that is not compatible with a simple set of instructions.