Mechanical Protection vs RCD Requirements

[SW1970]

Hi, being asked to look at an existing commercial setup where 3 large rooms are rented out to different commercial tenants (picture rooms 1, 2 and 3 adjacent and in order left to right). The point of supply is inside room 1 and is single phase with a main board. Room 1 is occupied by a tenant who is rarely there. There is a 30mA RCD + MCB for room 3 in this main board, and there is a sub board in room 3 that has MCBs only.

Problem is that the tenant in room 3 has to go to tenant of room 1 to switch RCD back on whenever it trips (e.g. light bulb blowing), and as they are not often there is a bit of mare. Tenant of room 3 has the public visiting so landlord has asked for a better solution.

Should be an easy job to change the RCD in room 1 to RCD-S with MCB and install standard CU with RCD and MCBs in room 3. However, the cable from main board in room 1 to room 3 is T&E and travels through room 2 to room 3 over a distance of about 30 metres. Inside room 2 it is exposed and within reach of the tenant most of the way (but is hidden in an enclosed column for a few metres).

The tenant in room 2 is a small business with 2 staff and the public does not visit. I think it's probably not acceptable to implement this due to the partially hidden cable and really the cable should be either upgraded to SWA or have mechanical protection around where it is hidden. Does that sound about right or is the "mostly visible" nature of this T&E ok?

Many thanks!

 

[telectrix]

depends on where the hidden cable is. if it's <50mm deep, e.g.plastered in, it's liable to be damaged by nails or screws etc. if it's out of the way, i would consider that no RCD protection is required.

 

[SW1970]

Thanks tele, what about if the visible section of T&E is at head height clipped direct along a wall? It's not really out of the way but is fully visible. Would you consider this also does not need RCD protection? Cheers!

 

[Deleted member 26818]

It would appear that the Installation never complied at the time of installation, or alterations have been made to the structure of the building such as to make the installation non-compliant.
Regulation 610.4 requires that it is verified that any addition or alteration complies with the Regulations and does not impair the safety of the existing installation.
As the installation is commercial, removing the RCD protection would comply with the Regulations, but doing such would impair the existing safety.
This leaves you with the option of either providing another means of additional protection which will provide the same degree of protection as would be provided by use of an RCD, or altering the installation so as the RCD is in an accessible location.
It will be for you to determine whether the use of an earthed metallic sheath (SWA) will afford the same degree of safety as would be provided by the use of an RCD.
However, it is still a requirement that the installation be divided into circuits so as to minimize inconvenience (Regulation 314.1), does having the CU in a room where users do not have ready access, comply with this requirement?

 

[Rauer]

Just a stab in the dark here but I thought sub mains don't require rid protection?
And I thought with it being commercial rcd's weren't nesacerily required

 

[rich.250]

Rcd protection is required for additional protection. In this case; e.g in wall less then 50mm or a TT system. Then dependant on readings you should decide on the necessary rcd.

 

[Rauer]

Exactly as in a commercial setting a skilled person is instructed to put up pictures ( apparently they are anyway!!!!)

 

[telectrix]

the same skilled persons who can guarantee to pierce a cable 1/2" wide in a 15ft wide wall. best cable finders i've seen are nurses and secretaries.

 

[Rauer]

Completely agree with that!!

 

[Deleted member 26818]

Just a stab in the dark here but I thought sub mains don't require rid protection?
And I thought with it being commercial rcd's weren't nesacerily required

That is correct, RCD protection is not required.
However as I stated in my post, it has been provided, which therefore means that to remove it would make the installation less safe.
Something that in itself is prohibited.

 

[Rauer]

Aha ,everydays a schoolday!!

 

[rich.250]

Would it necessarily make it less safe spinlondon?? As the protective device should still operate in 0.4 secs if the Zs is correct.
So all you would remove is the neautral to earth protection and in theory there joined at the tranformer.
Rcd protection could then be added for additional protection in the individual units for socket outlets etc where necessary

 

[Rauer]

Is it still 0.4 tho

 

[rich.250]

Good point, got ahead of myself. dependant on the earthing arrangement. tn systems the operational zs values are the same for 0.4 and 5 secs. Without seeing it im struggling to help. best advice is to check the Zs of the submains and look at table 41.* (depentant on device)

I totally agree with spinlondon you cannot make it less safe, although there must be another method of achieving the required disconnection time that will comply on the access and convienience sides.

 

[Deleted member 26818]

Would it necessarily make it less safe spinlondon?? As the protective device should still operate in 0.4 secs if the Zs is correct.
So all you would remove is the neautral to earth protection and in theory there joined at the tranformer.
Rcd protection could then be added for additional protection in the individual units for socket outlets etc where necessary

At present the circuits have RCD protection which should provide a disconnection time of 0.04s in the event of an earth fault.
Such a disconnection time would be achieved with a current as low as 300mA.
It would not be possible to produce comparable results using an overcurrent device.
Whilst it is possible using an overcurrent device, to provide disconnection times lower than 0.04s, such times can only be achieved using significantly greater values of current, than those required for an RCD.

 

[rich.250]

I see what your saying, never really thought of the amount of PeFC required for the disconnection time in this circumstance. Thanks spinlondon
In that case, the RCD will have to stay. :/

 

[SW1970]

Great input guys, so, the options are:

1) Leave RCD at origin in place (TN-S arrangement) - undesirable because of the inconvenience factor in room 3, or
2) Replace the RCD at origin with RCD-S and provide suitable mechanical protection along the cable and then RCD protection in room 3. Question here though: is mechanical protection required on the whole length, or just the parts that are hidden?

Cheers!

 

[Rauer]

Can you not change the RCd at the incomer for a main switch , extend the cables elsewhere and provide RCd protection from that point? I know this sounds easy on here and realise its not always that practical , am just throwing another idea in the air!

 

[rich.250]

As spinlondon said, the problem is you will be making the installation less safe due to the higher disconnection time /PeFC to operate device.
If it was a new install it would be possible providing the required disconnection time for the earthing arrangement was met for the submain.

 

[Rauer]

It won't be tho cause your just moving the RCd to another place , say for example the RCd is in the room at the mo , extend the main tails from the meter and extend the mains to the flat into a hallway for instance and add the RCd protection in there, then all existing cables will have RCd protection just make sure that your mains from the supply cables are either surface or buried more than 50mm