Maximum current through RCD.

[Plug]

Please forgive me as this isn't specifically related to domestic electrics, but is similar.

I'm going to be holding an event on a field in a caravan site, and the owners have given me use of an electrical supply. It is a box with two 25A 30mA RCD's, each feeding two 16A MCB's, each of which feeds a 16A IP44 Ceeform socket. Like this:

I understand how MCB's work, and the concept of a current flow imbalance tripping out RCD's, but can someone explain to me the significance of the 25A rating on the RCD's. What would happen if all the 16A outlets had full load drawn?

Thank you in advance,
Plug.

 

[Adam_151]

Its just the current carrying capacity of the RCD, in the same way as a switch will have a max rating, simple as that, it won't trip out, but rather get warm/start to burn up, etc (though the tolerances will probably actually prevent it from burning up in the real world)

If two 16A outlets are fed via each 25A rcd then the potential exists for overloading the RCD , maybe the designers of the unit decided that this was unlikely for the use it was designed for (but clearly if you are hanging say event lighting off it, then its quite possible for it to happen)

40A RCDs would have been a better choice IMHO

Edit: http://www.blue-room.org.uk has an electricial forum dedicated to queries to do with electrics for events, they punt queries about domestic electrics over here, so...

 

[Steve]

The RCD being rated at 25A - that is its rated maximum. If you draw more for a length of time, its contacts may weld shut and not open when the RCD trips. The RCD is not an overcurrent device, so it will not trip if you draw over 25A.

 

[Plug]

I'm beginning to think that the best thing to do might be to put a 10A MCB in an IP65 enclosure on one of the outlets from each RCD.

Edit: I mean on a cable out of the 16A socket, as I have no control over what's inside the supply unit.

 

[Plug]

I have seen lots of cables for sale on online auctions, sold as suitable for use when camping, which have a 16A plug for a Caravan type outlet, and then one or more non-RCD protected 13A sockets at the other end.

I'm reliably informed that BS7671, section 608, requires a 30mA RCD to be present at both the supply end, and the Caravan end of the cable supplying a Caravan. I'm not sure that I understand exactly why that is, but I am not going to challenge it.

I know that this regulation deals specifically with Caravans, and does not refer to Tents. I will be running a supply from one of the above pictured RCD'd outlets to a marquee (Which probably comes under BS7909), and I'm wondering whether or not to include an RCD at the marquee end (either individually on the 13A socket or in small consumer unit form).

I assume that safety would not be compromised by having more than one RCD in a system, but could do with confirmation of this from someone more 'in the know'.

I'm pretty sure that in a domestic install (not that it's at all similar) you would never have individually RCD'd outlets on circuits protected at the CU by RCD's. Is two RCD's in my case pointless?

Ta muchly.
Plug.

 

[Spark123]

It is electrically fine to have two RCDs of the same characteristics in a circuit, it is anyones guess as to which will trip first in the event of a fault.

 

[Pensdown]

I can see your dilemma. Using multi outlet 13amp trailing sockets plugged into the caravan posts seems fine to me and I would not bother with a second RCD as I can’t see what it would achieve.

You may find the supply to each caravan post is only 20amps and the site owner relies on diversity. This is quite common but normally the MCB for each van is only a 6A or 10A max. Due to the overall loading on the site I’ve never seen 16amps per van. You must be on one of the new “Super Pitches” I keep reading about.

 

[plugwash]

I'm reliably informed that BS7671, section 608, requires a 30mA RCD to be present at both the supply end, and the Caravan end of the cable supplying a Caravan. I'm not sure that I understand exactly why that is, but I am not going to challenge it.

two reasons i can think of.

1: RCDs in those enclosures are outside all year (yes i know the enclosures are IP rated but damp air can still get in and condense on cold parts) and in many cases don't get switched on and off very often. the result is they frequently stick!

2: Old caravans may not have RCDs and the same applies to old or foreign campsites (for similar reasons they specify double pole MCBs for caravan installs in case polarity is wrong) so it makes sense to spec them for both.

to the original poster: find out whats supplying that outlet box, you don't wan't to assume you can draw 50A total from it and end up tripping a breaker back at base.

 

[Plug]

I'm certainly going to investigate what's at the other end with the site owner.

For another part of the site, we're using a small (4kw) generator. I need to protect the generator from overcurrent, I'm pretty sure that the best way to do this is use a CU type box, with a 16A MCB, (feeding a 6A MCB for lighting, and a 16A RCBO for a weatherproof 13A outlet). Would this sort of box be what I need?

Thanks in advance
Plug.

 

[RF Lighting]

Yep that should be fine, providing you use apropriate glands for cable entries.

 

[Plug]

IP68 Stuffing Glands and 2.5mm^2 Arctic cable all the way.

 

[Spark123]

I'm certainly going to investigate what's at the other end with the site owner.

For another part of the site, we're using a small (4kw) generator. I need to protect the generator from overcurrent, I'm pretty sure that the best way to do this is use a CU type box, with a 16A MCB, (feeding a 6A MCB for lighting, and a 16A RCBO for a weatherproof 13A outlet). Would this sort of box be what I need?

Thanks in advance
Plug.

What earthing arrangements does the generator have?

 

[Plug]

What earthing arrangements does the generator have?

The generator will have an earth spike connected to it's frame. One earth for the whole system, making it TN-S (as far as I know). I'm planning to connect the frame of the (aluminium) marquee that the genny supplies to the same earth. Can you confirm this as correct?

I've changed my mind about the fusing from my post above, I realise that I need to start off with everthing under a 16A RCBO (16A to protect the 4kw generator from overcurrent draw). Anybody know of an IP65 enclosure that will take RCBO's?

The RCBO will then feed a 6A MCB for some lighting, and an IP55 13A outlet.

thanks guys.

 

[RF Lighting]

The enclosure in your earlier post should take RCBOs. If the generator earthing is from an earth spike then it will be a TT arrangement. If you are connecting the marquee frame to earth then it needs to be connected to the MET.

 

[plugwash]

Anybody know of an IP65 enclosure that will take RCBO's?

if you buy the two module wide type they should fit in any din-rail enclosure. The one module type can be more of an issue because of thier shape (to fit the RCBO in on module they have to extend it in another direction).

your earthing arrangements sound sane, earthing the marquee frame is sensible if you have cables attatched to it (which you likely will). Earth rods should be tested with a proper earth rod tester.

btw do you plan to run at night? if so then you really should make sure that any area is lit via two seperate rcds so that if any one rcd trips you don't have complete darkness (self contained emergency lights would be another way of ensuring this).