Outdoor Control Box advice (Earthing and design)

[Gaijin]

Hi guys,

I'm recently qualified so naturally a bit hesitant on my first jobs so looking for advice as much as possible.

I'm working as a junior at a domestic job where the customer has a large garage which has a sub main (run from the main unit in the house via SWA) consumer unit in it.

However he'd like to run power from the garage to 40 metres out by his driveway gates where a control box (painted metal IP rated enclosure fixed to wood posts) into which he wants to put a smaller plastic control box for an electric gate and also in which he wants a twin socket for if he wants to use power tools down by the gate etc. My boss says the gate motors are 580W (total for both) plus a small circuit board enclosed in the smaller control box.

Power is to be run via ducted SWA in the ground. The property is TN-C-S .

My boss has said it's an excellent opportunity to test my knowledge and design skills as there are a lot of variables, and so he has tasked me to come up with an approach on my own, before we return to site later next week after the SWA cabling has been laid.

My questions from my research:

Earthing:

I've read a lot about exporting PME earths in TN-C-S installations and from what I can see it's about 50/50 in the industry about whether I should TT the control box as in the event of a broken neutral, the enclosure could become live. The property is also a rural property and so I'm not sure whether the distance out makes the risk larger? So should we export the earth or should we TT the control box MET via earth rod? In the instance we would TT I would not earth the SWA at the control box end, only at the garage CU.

Design:

I'm thinking I would terminate the SWA at the control box end into a small DB in the control box with an 6A MCB for the gate (as a fixed appliance) and a 16A MCB for the twin socket. On the garage consumer unit I'm wondering what I should place to terminate SWA from the control box DB as I was taught that outside circuits should be RCD protected, and so I was looking to put in a 25A RCBO as the feed for the control box circuit in the garage board as there is currently no RCD's in the garage CU .

Also, I've considered for the gate circuit MCB the fact that it's running motors (surging). Am I correct that due to the motors being relatively small that Type B should be ok?

Thanks in advance

 

[westward10]

I would personally fit an rcd socket within the enclosure and connect from that a fuse connection for the gate which probably doesn't need rcd protection.

 

[Deuce]

Does the smaller control box power the motors? That would dictate the surge on the MCB if so, as it could mitigate the surge from the motors themselves in various ways.

Although given the very low wattage (for a set of motors), it's unlikely you will upset anything. Of course the ultimate fail for the customer would be if the gates did trip the MCB every couple of days - nothing looks worse than paying for electric gates then your neighbors seeing you getting stuck outside! So it's well worth looking closely at the motors and the controller, then a quick google to look at the controller datasheet and see what is likely to be passed back upstream (if anything) via the controller.

 

[Gaijin]

Sorry for the delay in coming back to you guys.

Does the smaller control box power the motors? That would dictate the surge on the MCB if so, as it could mitigate the surge from the motors themselves in various ways.

Yep, the control panel in the box inside the larger enclosure controls the motors over 24V I believe.

So it's well worth looking closely at the motors and the controller, then a quick google to look at the controller datasheet and see what is likely to be passed back upstream (if anything) via the controller.

Thanks - i've asked the customer for the details and will do some research.

Does anyone have any opinions regarding the earthing please?

Thanks for your time!

 

[Deleted member 9648]

Use the TNCS earth, the only time I would consider TT'ing anything external to the MET is if section 7 forbids it, or there are impractical main bonding requirements. Neither apply in this instance.

 

[Deuce]

Yep, the control panel in the box inside the larger enclosure controls the motors over 24V I believe.

One way or another the motor is going to be geared and almost certainly soft start. So inrush current is mostly mitigated anyway.

Also is there a battery? If so the motor will just take whatever current it needs from that and nothing upstream. The charger itself may cause a surge however, depending on how it operates.

When you get into it, just look at the components and figure out which is actually applying the load directly to the circuit. Once you have that figured out just take the model number(s) and Google to reveal what you're really dealing with.

I would be surprised if there was any significant inrush to be honest. It would be odd for the company that designed the system to not account for demands within their own control circuitry. You should do the research and calculation just for the sake of your own knowledge and intrigue, but I'd be very surprised if you ended up needing more than type b. The reality is that the biggest load on the new circuit is likely to be the guy plugging a mower/trimmer into the sockets you're providing, not the gate control/motors.

I would use a 25a rcbo in the garage too, although there is an argument that in theory he could plug in 2 x 13a mowers in at once and have the gate operate at the same time... In which case 32a. Personally I believe it's sometimes easiest to just ask what the likely usage is to choose the safest level of protection without causing inconvenience.