Time delay RCD - BS7671

While I agree that the current wording in BS 7671 isn't clear on that point - the previous references to operating within 40ms at 5x IΔn having been removed - the new references to acceptable standards (BS 61008 etc - see section 531.3.4) have pretty much the same effect - those standards typically specifying that a 30mA RCD must operate within 40ms at 150mA (or sometimes 250mA). Similarly I think you'd have problems sourcing a 30mA RCD to recognised standards that incorporates a delay - even the adjustable ones seem to disable the delay mechanism on the 30mA setting. In any event you can't reliably discriminate between two 30mA devices on time alone - as both will have a wide range of possible trigger currents (anywhere between 15mA and 30mA) a residual current within that tolerance range could trip trip the delayed unit (after the delay) but not cause the undelayed unit to trip.


In your case, if you have one RCD covering the whole board, would it not be just as easy to locate it at the start of the submain (and change the RCD incomer to a plain switchdisconnector)? Either that or substitute a SWA cable for the soft sheathed one (if needs be biting the bullet and re-doing some construction work), or change the run to surface mounted (in trunking say) for that part of the run.


  - Andy.

I would have to second the metal trunking idea.

Even if you had to install an appropriately sized metal clad isolator at high or low level perhaps and cut out/disconnect and abandon the 30cm that currently runs through the wall (The internal parts of which are metal), assuming that the part that is in the wall is the last little bit? If its in the middle of the run, it gets a little harder. I assume aesthetics are also important?


Where in the run is the 30cm problem bit?

The wall cannot be altered. This is the first thing I have asked.

It is not on the table as an option.

Unfortunately.

The feeder is like 40m in lenght running in a cable basket and then drops into a wall partition for 30cm to meet with the back of the CU.

The problem is that when I designed it I considered only the 40 meters on the cable basket however I did not realise that for the last part it needs to go into the wall partition.

An now I MUST use an RCD and I am looking if I can use a time delay one.

Alternatively, the easy solution is to take out the RCD incomer of the CU and use a regular 30mA RCD int he beginning of the feeder.

Cheers

What sort of cable? T&E?


If it's in basket, then is it accessible before it drops into the partition - though the ceiling for example? If so can the last 30cm be pulled back and replaced with SWA for example?


  - Andy.

What size of supply are we talking about? 100Amp three phase or 1000Amp 3 phase? and what size and type of cables are in use? (This will let us know how easy or hard that last little bit may be to remove from the wall and DB without damaging the wall)


In basket and not SWA - have you used the HORNY 7 cable? (H07RNF) in which case it should be fairly easy to withdraw?


If you cant withdraw the last 30cm of run, Is the aesthetics particularly important to where the DB is and do you have the space to fit an isolator such as   or a suitably rated "connector block" in a metal clad box in order to lengthen the bit to get down to the board in metal trunking that runs down the side of the DB. I think locating the RCD in the feeder pillar will cause future headache galore. If the supply is small enough you could even use henley blocks.

Can you give us an example of a time delayed 30 mA RCD?


If you cannot you can forget that idea.


Andy B.

At least in principle, you could, so long as with the delay it was still inside the 0,4 or 0,2 seconds required for ADS to operate depending if TN or TT.

50- 100mS maybe OK.

Now

Some earth fault relays can be set up to a threshold of 30mA and small delays -

BUT I caution that is not the normal problem they are used to solve, and once you have seen the price of that method I fear that not only will cutting the wall be back on the table, so will most of a complete rebuild.  Or deciding it is an acceptable non-compliance for reasons that may be hard to justify, or trying to slip some armour-flex over the top of it from the free end, or covering that tricky 30cm hop with an access panel ...

this is just the earth fault detecting part. Sensing transformer and suitable contactor or aux coils to fire an MCB are extra !   


Another example 

Apostolos Kasinalis:

..

Alternatively, the easy solution is to take out the RCD incomer of the CU and use a regular 30mA RCD int he beginning of the feeder.

Cheers

 I might be tempted to leave the old RCD in. If someone has to go 40M to reset it every time it trips, that is definately inconvenient. Leaving the old one in as well as the new one might mean that 50% of the time it could be reset locally, less inconvenient.