if max zs is exceeded

[muffin]

back on topic then please.

im a time serverd spark and this has been ticking through my head for a while now, as we have said a tt system relies on the rcd giving protection where the zs is too high.

saying this is unacceptable for tncs but is ok for tt seems hard to understand. now the 17th ed is in force and most boards have rcds covering all circuits the question has to be asked do we need to "worry" about zs as much. when i say this i mean should the regs be changed to allow a ball park for tncs e.g within x% of the ze. a type d 32 amp 60898 has a very low zs of .375 ohms. it does not take a long circuit to exceed this.

our nic guy pointed out a good case he came across the other day where the ka on a tt system was very low. so even on short circuit the device would not trip as there was not the current available to do so.

andy

 

[Megaohm7]

If someone is dedicated enough to want to do well in this industry then good luck to them. By being dedicated is a willingness to learn.

 

[Chr!s]

back on topic then please.

im a time serverd spark and this has been ticking through my head for a while now, as we have said a tt system relies on the rcd giving protection where the zs is too high.

saying this is unacceptable for tncs but is ok for tt seems hard to understand.

Andy, its perfectly acceptable to use an rcd for fault protection on a TNCS system. though you still need to protect the cables against short circuit.

now the 17th ed is in force and most boards have rcds covering all circuits the question has to be asked do we need to "worry" about zs as much.

Well i think you need to look at the type of system employed, if i was to have tncs with a high zs id be worried, its either the supply at fault or the installation and either way needs correcting as soon as, there may be no earth soon if the fault persists.

when i say this i mean should the regs be changed to allow a ball park for tncs e.g within x% of the ze. a type d 32 amp 60898 has a very low zs of .375 . it does not take a long circuit to exceed this.

Again, this comes down to how the distribution is engineerd, if engineered correctly and maintaind, low values are not an issue, so any high values of ze on a tncs need investigation by the dno.

our nic guy pointed out a good case he came across the other day where the ka on a tt system was very low. so even on short circuit the device would not trip as there was not the current available to do so.

If the Pscc was that low how can the System comply with the regs, although no time is given for short circuit the k values are for 5 second disconnection so how are they going to prove compliance?

If the tt does have a low Pscc then again why? Voltage drop could be an issue and indicates a problem somewhere which needs further investigation.

Well this is the other point, if our ze is high dependent upon where the fault lies, because the earth and neutral are combined in a tncs then theres going to be a large voltage drop across the live conductors.

andy

 

[muffin]

my example of a max zs of .375, lets say you have a ze of .34 or. 35 either is fine that only leaves .035 ohms not very much at all.

 

[ADS]

my example of a max zs of .375, lets say you have a ze of .34 or. 35 either is fine that only leaves .035 ohms not very much at all.

I would say this would be an example of where you would 'knowingly and deliberately' use an RCD for earth fault protection - nothing wrong with that.

The circuit would possibly be designed with that in mind.

That's a world-apart from having a high Zs on a TN system, where the design specs say that it should comply, and accepting this as OK because there is RCD protection on the board.

As to your other point - I don't think it would be a good idea for us to start totally relying on RCDs for 'earth fault protection' - they work in a different way to MCBs and can malfunction.

I'm happier, on a TN, with the MCBs used as they are and the RCDs used as additional protection.

 

[Chr!s]

my example of a max zs of .375, lets say you have a ze of .34 or. 35 either is fine that only leaves .035 not very much at all.

Andy

DNO Guidance

5.4.4 Provision of PME terminal

There is no maximum limit of loop impedance for a PME supply defined in either the current edition of BS7671 'Requirements for Electrical Installations' or Electricity Association Engineering Recommendation P23/1.

The values of loop impedance in Electricity Association Engineering P23/1 are indicative values for the majority of installations whether they use PME or other forms of earthing. They are not maximum values permitted for provision of PME.

So this in effect allows higher ze values than the .35 ohms if say there's a 1361 60 amp fuse BS7671 requires a value of .70 ohms for 5 sec disconnection.

 

[m4tty]

I would say this would be an example of where you would 'knowingly and deliberately' use an RCD for earth fault protection - nothing wrong with that.The circuit would possibly be designed with that in mind.

That's a world-apart from having a high Zs on a TN system, where the design specs say that it should comply, and accepting this as OK because there is RCD protection on the board.

As to your other point - I don't think it would be a good idea for us to start totally relying on RCDs for 'earth fault protection' - they work in a different way to MCBs and can malfunction.

I'm happier, on a TN, with the MCBs used as they are and the RCDs used as additional protection.

Can you explain abit about the bolded bit above please mate. Interested why mcb's are never tested but rcd's are? cheers

 

[Andy™]

Can you explain abit about the bolded bit above please mate. Interested why mcb's are never tested but rcd's are? cheers

its easy to test an RCD. its not so easy to test an MCB.

and even harder to test a fuse

 

[ADS]

Can you explain abit about the bolded bit above please mate. Interested why mcb's are never tested but rcd's are? cheers

Quite scary that you should ask that question because it's exactly what I thought about after I typed that post.

I know the reason that MCBs aren't tested 'in -situ' is because the currents involved would be much to dangerous.

I'm guessing manufacturers test so many per batch, in a controlled situation - and for the rest we just pray

As for the necessity of testing every RCD (and the thoroughness of testing them) - the opinion I form from this is that they may be more susceptible to faults.

Think of it this way

MCBs are made to a British/European Standard - BS EN 60898

So in other words an MCB to this standard will have certain performance characteristics and, as you say, doesn't require testing at installation.

An RCD also has a British/European standard (61008) and, therefore, also has minimum operating requirements - yet these operating requirements need to be tested on installation???? What's that all about?

As I said, RCDs for additional protection only - unless there's a good reason for being unable to get the Zs low enough.

My opinion, only, by the way.......others may feel differently. ; \

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