Melting rcd

[daniel hill]

Afternoon, had a fault this afternoon at a property that one of my lads rewired 12 month ago. The property in question has a 11 kw in line heater that warms a tank up for the domestic hot water and also the central heating. The heater draws close to 50 amp so we fitted a separate consumer unit, tails, henley block ect. The rcd was a Wylex 80A 30mA with a 63A type b mcb. The circuit is wired in 16mm t+e all surface mount and no signs of burning on the cable ect. The problem that I have is this afternoon I have fitted a new rcd consumer unit (Schneider electric) with again a 80A rcd 30mA and 63a mcb fitted. After half a hour I started feeling heat on the rcd and the smell of burning. I have isolated and had to remove as needing to think what is causing the heat. I made sure that all the connections were tight. The voltage of the install is 243V tncs. Have the amp meter on and is only drawing 46A so it's not overload. Any one got an idea of the cause of the burning. Hope you can help, thanks Danny

 

[GLENNSPARK]

test the circuit insulation etc and increase the curent of the RCD , although the current drawn is below the rated current if the contacts are slightly weak then it can over heat at high currents , i would always go for the higher rated RCD in situations like this so go for either 80 or 100 amp RCD dont forget these are not overload devices they are RCD's basically an isolator with RCD protection

yep. any imballance over 30mA and it will go (if its working ok)..lol........

 

[GLENNSPARK]

you`v got a 63A MCB for overcurrent protection here and any signs of overheating there?.....If thats happy then it has to be crap contacts within that RCD......like Nick says..go up a size......give it every chance.....

 

[daniel hill]

Yep have checked all the cables were in the clamp, tightened up by 2 electricians and a test engineer, am going to try a 100A rcd tomorrow and also going to take a thermal image camera. I just carnt understand why a rcd rated at 80A is running warm when the load is 47A a load difference of 33A under maximum current draw.

 

[Engineer54]

Yep have checked all the cables were in the clamp, tightened up by 2 electricians and a test engineer, am going to try a 100A rcd tomorrow and also going to take a thermal image camera. I just carnt understand why a rcd rated at 80A is running warm when the load is 47A a load difference of 33A under maximum current draw.

That's a good and wise idea to make use of a thermal imaging camera. It will certainly show you, if and/or where there is any high resistance causing your overheating. Try it first, with the existing RCD in place, before moving on to the higher rated unit, if just to confirm where the problem lays. Check the whole of supply side of things too!!

 

[Knobhead]

It’s no reflection on you Daniel.
It’s the usual cost cutting by manufacturers. If they can shave a gram off the contacts they will. They work on the basis that the loading won’t be anywhere near the nominal maximum.

A sad state of affairs but to undercut the competitors, quality goes out of the window.

 

[malcolmsanford]

I maybe well out here and not knowing the setup but something like this sounds like an harmonic problem

http://www.copperinfo.co.uk/power-q...ral-sizing-in-harmonic-rich-installations.pdf

"This issue is particularly important in low voltage systems where harmonic pollution by single phase loads is an
increasingly serious problem. Triple-N harmonic currents add arithmetically in the neutral conductor
rather then summing to zero as do balanced fundamental and other harmonic currents. The result is
neutral currents that are often significantly higher, typically up to 170%, than the phase currents"

The above is an extract of the document in the link concerning this and with such a large load I wonder if your getting problems and conflicts within the installation

 

[spark 68]

Hi Malcom,

How do you check/test for Triplen harmonics ?, would just a normal clamp meter around the N conductor show this ?, as they will be @ 150Hz

Iam aware of Triplen harmonics, and how they are cumlative/additive, in the Neutral,and have indeed built switchgear where the N bars were rated @ 200%, usually for data centres etc. with lot's of SMPSU's, but I never designed this stuff (above my pay scale), and have never tested for this either.

TIA,
Spark

 

[malcolmsanford]

As I work in the IT industry over here all my installation are designed using large cable sizes, all the power cables are minimum 6mm and we have limited the amount of sockets to 4 doubles on this. Also we try to limit the distance and any power circuit over 15mts from the source will be run in 10mm and then back down to 6mm. This is for conventional offices. For Data Centers or server rooms we normally use power track systems.

We only normally check the 3 phase boards for harmonics using a Power Quality Analysers it does what is says on the tin it analyses the installation, in the same way as you would fit the moniter for overvoltage it will give you the frequency data on the installation.

 

[Engineer54]

While Harmonics is generally a very plausible possibility in commercial applications, i think it's unlikely in this situation. As the load this RCD is looking at, is basically a purely resistive heating load in a domestic property.

 

[i=p/u]

i say try hager..

 

[malcolmsanford]

While Harmonics is generally a very plausible possibility in commercial applications, i think it's unlikely in this situation. As the load this RCD is looking at, is basically a purely resistive heating load in a domestic property.

I would agree E54 but as I posted not seeing the set up not sure if there are any inductive loads on the system. Logic would tell us it should be purely resistive, but 2 protection devices burning out when they are not overloaded in the conventional way points towards really poor devices or something causing a massive overload situation, as you would get on the neutral through Triplen Harmonics.

Easy way to find up up the CSA of the cable

 

[daniel hill]

Just to let you all know, I have been back to day armed with a 5 way db with a 100A 30mA main switch and a 50A mcb at first when I fitted it the mcb this time was getting warm on the side. Around 40 deg with the camera. This was only after say half a hour. Anyways after a few discussion we have decided to remove the cover from the heater and remove one of the four heater elements. Dropping the load on the circuit down to just 39A. After a hour running the mcb got no hotter than 26 deg at this I was happy that nothing was going to burn out at all, as most said I think the gear out there is not fit for purpose. Never had anything like this on the rewireable fuse boards with the 2 screws per each tail. Thanks all for your advice. Danny