Would the S type RCD not need to be at source? You imply it is to be at the end of the cable it is protecting. This would not work. And again, usually armoured cable or metal conduit would be used to run the cables, thus they dont need RCD protection at all. And if they did, the 17th requires 30mA, not 100mA, nor S type.
RCD Tripping Problem
- Thread starter dan86
- Start date
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tonylec
You are right it needs to be at the source which is what I meant. It has to be 100mA typeS or it will trip out every time the RCBo trips and take the lot out (ie: it won't cure the problem). We fit the 100mA or occasionaly 300mA RCD to comply with reg 532.1 and to give additional RCD protection to the cable run in school and commercial premises, these runs are in armoured. You could alternatively run from a red link to another local red link in concentric.
Guys,
Thanks for your comments. I am considering paying for a seperate circuit to be installed for the computer stuff as its very important to us. Basically it needs to be seperated from the household stuff but still protected to comply etc.
I am also looking at something called an isolation transformer and whether or not that could be of use.
Thanks for your comments. I am considering paying for a seperate circuit to be installed for the computer stuff as its very important to us. Basically it needs to be seperated from the household stuff but still protected to comply etc.
I am also looking at something called an isolation transformer and whether or not that could be of use.
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tonylec
Sorry if I didn't make what I was alluding to clear, take this example of a recent instal.
New office building on school premises, fed from sub DB in boiler house via a 100mA time delayed (type S) RCD, this protects anyone who inadvertently cuts the cable when digging and complies with the requirements for wiring in educational premises (a DfEE/HSE regulation, not a 17th one). at the other end the armoured cable enters a memshield DB with RCBo's on each circuit giving 30mA protection, the 100mA one needs to be time delayed to prevent it operating at X5 and causing a loss of power to the building.
alternatively another scenario is a new DB on the third floor for an IT suite, the mains is separated in the switchroom from a henley phase block and then fed to a red link (a cut out unit coloured red) fitted with a 100A fuse and from here to the third floor in 25mm2 concentric cable with an external earth, at the other end there is a red link with a brass dummy fuse in to act as a disconnector, tails feed into the DB and the subcircuits are on RCBo's
New office building on school premises, fed from sub DB in boiler house via a 100mA time delayed (type S) RCD, this protects anyone who inadvertently cuts the cable when digging and complies with the requirements for wiring in educational premises (a DfEE/HSE regulation, not a 17th one). at the other end the armoured cable enters a memshield DB with RCBo's on each circuit giving 30mA protection, the 100mA one needs to be time delayed to prevent it operating at X5 and causing a loss of power to the building.
alternatively another scenario is a new DB on the third floor for an IT suite, the mains is separated in the switchroom from a henley phase block and then fed to a red link (a cut out unit coloured red) fitted with a 100A fuse and from here to the third floor in 25mm2 concentric cable with an external earth, at the other end there is a red link with a brass dummy fuse in to act as a disconnector, tails feed into the DB and the subcircuits are on RCBo's
What happens in the event of a fault putting neutral to earth on a final circuit?
tonylec, I'm curious.
Your first example, you have an RCD at source of submain due to a DfEE/HSE requirement.
Your second example, you dont.
Why is this?
In either case, its not required by the 17th edition, and I dont know why either of the above authorities would make such a requirement. As soon as you cut into a phase conductor in an armoured/concentric cable, the fuse/MCB protecting it will operate. Madness.
Your first example, you have an RCD at source of submain due to a DfEE/HSE requirement.
Your second example, you dont.
Why is this?
In either case, its not required by the 17th edition, and I dont know why either of the above authorities would make such a requirement. As soon as you cut into a phase conductor in an armoured/concentric cable, the fuse/MCB protecting it will operate. Madness.
SWA supplied by a fuse / MCB / MCCB etc would acheive this.
Which regulations specifically are these, and who says they need to be followed?
Why is there any need at all for RCD protection?
That RCD will still trip if there is a N>E fault anywhere on the installation.
Whats all the messing about with henley fuses for? Why not feed the supply cable directly into the DB?
Is straight concentric cable recognised by BS7671 for use on a private installation?
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tonylec
Well i'll try and answer best I can - not being at the office having to work from memory. To start we only do schools so thats our field of expertise and we are fully qualified so i'd like to think we know what we are doing, there is a document published by the dept of education outlining various requirements for the wiring of school premises amongst many other tech spec requirements, this is fed down to us in the basic specs for the jobs by the school maintenance officer- RCD is king in the minds of those making up the specs, everything new is RCD protected and any length of cable is protected likewise.
The redlink arrangement is not covered in the 17th edition as it falls under ESQCR as it is the property of the supply authority until the final link, however there is no good reason not to use such a set up on a private line and such is common in flats where the main meter is owned by the board and sub meters are owned by the landlord.
The redlink arrangement is not covered in the 17th edition as it falls under ESQCR as it is the property of the supply authority until the final link, however there is no good reason not to use such a set up on a private line and such is common in flats where the main meter is owned by the board and sub meters are owned by the landlord.
I don't understand the advantages of this system - if you have to run a separate earth along with the concentric, exactly what are the advantages over installing a switchfuse at source and running in split con?
Around 50% of my work is schools, in at least 5 different LEAs and we have never been asked to work to anything other than BS7671.
Have you never pointed out to the people who give you these specs quite how badly designed and non-compliant their installations are?
Have you never pointed out to the people who give you these specs quite how badly designed and non-compliant their installations are?
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tonylec
Got to be honest and say i'm quite happy with the specs and they always pass muster with the scheme inspectors when they pop there head round the door - so if the LEA is happy, the leccy board is happy, it works, it's safe, it meets the relevant BS standards, we get paid - whats to concern myself about!!
Anyway we've drifted way off topic, looks like the OP has a solution so i'll bid you gents good night!
Anyway we've drifted way off topic, looks like the OP has a solution so i'll bid you gents good night!
We do have a tendency to do that, but as we don't have an equivalent of the Combustion Chamber, there's nowhere else for us to discuss such little niggles!
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