tt system question

[sparky114]

Chaps..

Simple question..
Been studying and been reading peoples issues that arise on jobs ect i came across on one where one chap said that a rcd main switch can only be used on a TT system? Is this true and if why?

 

[telectrix]

no, you would use an RCD on a TT system to provide fault protection as the Ze on a TT is usually too high for a fuse/MCB to trip in the required time.

 

[TJ Anderson]

It is right in so much that it is only TT where you would have one RCD protecting the whole installation, although it would be a Time Delayed type and would discriminate with downstream RCD's.

 

[NDG Elecs]

I am fairly sure a 100mA S type could be used as a main switch on non TT systems. Only reason I could think to use one on such is for additional additional protection though!! As the 30mA's would provide additional protection after ADS with good loop readings. I would lean towards using a S type main switch on TT installs despite having dual 30mA also, as RCDs are a tad prone to failure. Although customers often dont comprehend the benefit.

I would be interested to know whether they are definately permissible.

 

[NDG Elecs]

114, this is from the beama RCD handbook:

 

[Richard Burns]

If you have an upfront RCD e.g as a main switch then you are providing a protective device that will take out the whole installation due to a fault on one item, which is not recommended (314 division of installation) because it will cause nuisance and possible danger, e.g. light bulb whole house loses power.
For a TT system all circuits will need RCD protection if the EFLI is too high for an MCB to operate.
An easy way of doing this is to provide an upfront RCD, but this causes problems with 314 so separating circuits for instance with RCBOs is preferable.
However if there are circuits that do not require additional protection with 30mA RCDs then the provision of RCBOs can be excessive so for a simple installation an upfront 100mA type S RCD will provide the ADS for non 30mA RCD protected circuits.
However if additional protection is required no time delayed RCD can meet the 40ms disconnection time required as the time delay is ~150ms and 100mA is great a current to provide protection of persons so they are not suitable for this purpose.
Generally 30mA RCDs do not come with a time delay.

 

[NDG Elecs]

Mine wasn't a typo; I did put additional additional protection in purposely! Obviously being a misnomer it is senseless!

Good point about the time delayed and 100mA not being suitable for AP. But diss. times for TT are 0.2 and 0.1 are they not for <\>32A ccts.

 

[Richard Burns]

I did notice your cavalier treatment of the words of wisdom from BS7671:cry_smile::sad_smile:!

A 30mA RCD used for additional protection must disconnect in <=40ms at 150mA (5Idn).

For a TT system the ADS times are 0.2 s and 1 s as you almost say and even a 100 mA TD RCD can meet the 0.2s so long as its trip time is less than 49ms without the 150ms time delay (for a total of 0.199 s)

I probably should have said trip time rather than disconnection time; me also being cavalier!

 

[NDG Elecs]

Sorry meant 1.0 not t'other way round.. Got 181.2mS on an S type last week. Where is info available on the actual preset time delay of TD RCDs. I presume this lies within MI's as never knowingly seen reference to it in BYB, OSG or GN3 etc... On the ones I fitted last week there was no indication on the Wylex blurb about 150mS or otherwise.

 

[Richard Burns]

Good point there, I may have entirely made that up?, I am not sure where 150ms came from, in looking in GN3 an RCD may trip from 130ms to 500ms to comply with the time delayed RCD limits, this would preclude a 150ms time delay. I definitely had in my mind that the expected time delay was 150ms, it may be in the BEAMA RCD handbook. I will check later.