TT Question

[bigd]

Dear All,

Had a heated discussion with another electrician today regarding TT installation. So your advice would be greatly appreciated.

Does a consumer unit (installed as part of a TT systems) with ALL circuits protected by 30mA RCD's require an additional 100mA RCD on the incoming tails? The consumer unit has a main switch and 2 30mA RCD's.

I know its normal practice to replace the main switch, in the consumer unit, with a 100-500mA RCD (Usually s-type time delayed), but this wasn't possible due to the design of the consumer unit (the manufacturer didn't have this option - a lesson learnt here stick with well known makes wylex, MK etc)

I also realise that any dedicated circuits for smoke alarms, medical equipment etc would require me to fit a 100mA RCD in the incomer, but there were no dedicated circuits just those protected by RCDs

The other electrician disputes this, saying all TT installations need a 100mA RCD.

Your thoughts?

One thing I did think is that where the consumer unit main switch feeds the 2 x RCDs the cables are considerably smaller than the incoming tails so the RCD feed cable could fail before the 60A DNO fuse. But then again the consumer unit is rated at 100A and the RCD's at least 63A, so I would expect the cable to survive. Am I thinking on the wrong lines here?

Dave

 

[Moses]

Dear All,

Had a heated discussion with another electrician today regarding TT installation. So your advice would be greatly appreciated.

Does a consumer unit (installed as part of a TT systems) with ALL circuits protected by 30mA RCD's require an additional 100mA RCD on the incoming tails? The consumer unit has a main switch and 2 30mA RCD's.

I know its normal practice to replace the main switch, in the consumer unit, with a 100-500mA RCD (Usually s-type time delayed), but this wasn't possible due to the design of the consumer unit (the manufacturer didn't have this option - a lesson learnt here stick with well known makes wylex, MK etc)

I also realise that any dedicated circuits for smoke alarms, medical equipment etc would require me to fit a 100mA RCD in the incomer, but there were no dedicated circuits just those protected by RCDs

The other electrician disputes this, saying all TT installations need a 100mA RCD.

Your thoughts?

One thing I did think is that where the consumer unit main switch feeds the 2 x RCDs the cables are considerably smaller than the incoming tails so the RCD feed cable could fail before the 60A DNO fuse. But then again the consumer unit is rated at 100A and the RCD's at least 63A, so I would expect the cable to survive. Am I thinking on the wrong lines here?

Dave

If you have a CU with 2 x RCDs and no non-RCD circuits available in the CU,
then for a T-T system, there is no need for an s-type 100mA RCD at the front-end.

 

[bigd]

Does the fact that the consumer unit is a an all metal enclosure or Class II insulated make any difference?

Dave

 

[PC Electrics]

Dear All,

Had a heated discussion with another electrician today regarding TT installation. So your advice would be greatly appreciated.

Does a consumer unit (installed as part of a TT systems) with ALL circuits protected by 30mA RCD's require an additional 100mA RCD on the incoming tails? The consumer unit has a main switch and 2 30mA RCD's.

NO

I know its normal practice to replace the main switch, in the consumer unit, with a 100-500mA RCD (Usually s-type time delayed) really?, but this wasn't possible due to the design of the consumer unit (the manufacturer didn't have this option - a lesson learnt here stick with well known makes wylex, MK etc)

I also realise that any dedicated circuits for smoke alarms, medical equipment etc would require me to fit a 100mA RCD in the incomer, but there were no dedicated circuits just those protected by RCDs again, really?

The other electrician disputes this, saying all TT installations need a 100mA RCD.

rubbish

Your thoughts?

One thing I did think is that where the consumer unit main switch feeds the 2 x RCDs the cables are considerably smaller than the incoming tails so the RCD feed cable could fail before the 60A DNO fuse. But then again the consumer unit is rated at 100A and the RCD's at least 63A, so I would expect the cable to survive. Am I thinking on the wrong lines here? yes

Dave

I suspect you and your friend are confusing the requirements of 16th and 17th editions.

 

[Moses]

Does the fact that the consumer unit is a an all metal enclosure or Class II insulated make any difference?

Dave

It shouln't make any difference, Make sure the IP ratings are complied with,
and check the earth bar, it should already be bolted to the metal chassis of the CU.

 

[johnboy6083]

Actually, if the CU is of metal construction, then really the tails should be protected by an RCD, in order to allow for ADS, if they short onto the metal case of the CU. I think a case may be made for not having an RCD, if a plastic bush is used (both tails through the same hole too), but personally, i would use an insulated CU.

 

[bigd]

Thanks for your advice chaps

 

[Spudnik]

AFAIAC The only time an rcd is needed on the tails is when a metal cu is used.

If the cu is all insulated there is no need.


Sent from my iPhone using Tapatalk

 

[bigd]

Does anyone know what the old 16th regs used to say about TT systems & RCD's? I'm guessing this is where the 'advice' I am getting may have come from.

 

[Guitarist]

As has already been said, 100mA s-type upfront if Metal enclosure, otherwise the 30mA RCD's are all that is required. That said, never hurts to have a backup RCD on TT systems, but not required.
The other electrician may be getting confused by the on-site guide which shows all circuits protected by a 100mA RCD, along with certain circuits 30mA protected.

 

[spark 68]

Does anyone know what the old 16th regs used to say about TT systems & RCD's? I'm guessing this is where the 'advice' I am getting may have come from.

From the 16th Ed. amd1 green OSG:

RCD's there are a number of instances where an installation is required to incorporate one or more RCDs. These instances include:

i) Where the Earth fault loop impedance is too high to provide the required disconnection time eg. where the supply authority does not supply an Earth-TT systems
ii)On Socket outlet circuits on a TT system
iii) On all socket outlets that may reasonably be expected to supply portable equipment outside.
iv) circuits supplying portable equipment for use outdoors supplied by means of a flexible cable.

It then goes on to say:

reg 314-01-01 requires installations to be divided into circuits to minimise inconvenience in the event of a single fault affecting blah blah. (see Regs 314 of current regs, it is very similar).

30mA RCDs installed to provide protection to socket outlets for outdoor equipment (see above) blah blah.

Where an RCD is fitted only because the earth loop impedance is too high for shock protection to be provided by an OCPD , for example in a TT system, the rated operating current should not be less than 100mA
If two RCDs are installed they should preferably control seperate circuits or a time delay 100mA RCD or S-Type should be installed.

As now under the 17th ed, almost all domestic circuits require 30mA RCD protection, and with split boards usually having two 30mA RCDs fitted the above is moot anyway.

 

[Engineer54]

As RCD devices are the primary means of EF protection on TT systems then it would be very prudent to have a 100mA S type up-front RCD, no matter whether the CU/DB is of metal construction or not!!

I don't like dual RCD CU's at the best of times, they are nothing but a compromise and can be a nightmare to fault find!! On a TT system you are relying on a single RCD device to protect several circuits, which isn't very clever when they are not the most reliable of protective devices, and only a 200 ohm Ra and whatever gas/water parallel bonding is giving you. (nothing where the incoming services are by means of modern plastic pipes) .

Personally, and professionally it is far better on a TT system, to go with an all RCBO (and preferably DP RCBO's) with an up-front addition protection S type RCD, as wirepuller states, it's going to be unlikely that 2 RCD's are going to fail at the same time...