Installing 100mA RCD For Sauna

[strake]

First, hello and thanks for such a great site. I've had a search for answers to this question with no luck but if I've missed something, sorry in advance.

Just moved into a house with a sauna in the summerhouse. Two mins after switching on the stove, the RCD (dedicated to all summerhouse circuits) trips on the CU.

Advice from Nordic (manufacturers) is moisture in the elements caused by limited use. Two solutions proposed..1. Bake the units in an oven for two hours. 2. Change RCD from 30mA to 100mA.

Tried the first, did not fix the problem.

So, question..is the replacement for a 100mA a straight swap on the CU and is the suggestion sensible?
Thanks again.

 

[jj4091]

I would have thought you needed to have it tested properly before you start replacing parts. There could be a fault on the element that causes it to short when warm. Do you have a multimeter to resistance check the element when it is warm(but with power off).Is the sauna on a separate mcb?Changing the rcd could resolve the problem but it may be a bit premature.

 

[strake]

jj4091, thanks for your reply.

Yes, I went through all circuits before getting on to the suppliers..that's why I couldn't understand how I was getting a short. Apparently, this moisture thing with the heater elements is quite common and the fix is to up the RCD.

There is a small CU in the summerhouse fed (via RCD) from the main CU in garage. The small unit has an MCB for the sauna and further ones for heating lights etc.

 

[sm1thson]

Two mins after switching on the stove, the RCD (dedicated to all summerhouse circuits) trips on the CU.

what other circuits are in the summer house? for example i would expect there to be a light, if steam has got in the light (likely) then there will be some leakage here so a little bit of extra leakage from the stove may just be sending it over the edge. if theres a socket circuit (presumably not in the sauna but maybe an external socket on the wall for the lawnmower etc) the same applies, and moreover you must also note that the sockets must be protected by a 30ma rcd not a 100ma.

 

[strake]

Yes, light circuits and power circuits but I think the problem is solely down to the heater.
I've been thinking about the problem while writing and possibly the next stage is to run the heater with the coals out and earthstrap removed to give the elements time to glow hot. Theoretically, that should dry-out any moisture or show-up any element with a problem.

 

[Steve]

Earth is there for your protection. Under NO circumstances remove it.

Im not sure if the elements need RCD protection. If not, you should remove the main RCD and protect the rest of the installation with RCDs as nessecary, leaving the sauna without. But only if the sauna doesnt require RCD.

 

[strake]

Earth is there for your protection. Under NO circumstances remove it.

I'm not intending to USE the sauna without an earth but I can't tell which element is shorting if the RCD keeps tripping after 2 minutes.

To answer your second point, I think an RCD is important because of the continued presence of water.

 

[Spark123]

Do you know a spark who can do an insulation resistance (AKA Megger) test on the elements for you?

 

[securespark]

I am surprised that the company advises replacing the 30mA RCD to a 100mA. This will not give you personal protection.

 

[JohnD]

A safer option would be to have one 30mA RCD for the sauna only (feeding no other circuits) and another for all the shed sockets.

This is very easily done if you can use RCBOs but would be a good move anyway. It woulkd be easier if the supply from the house was not RCD protected, and you had your RCD in the outbuilding.

It is not unusual for several circuits or appliances to have individual small leakages, which are individually not dangerous, but added together can reach the trip current. Outdoor and outbuilding fittings are often a bit damp, and steam from the sauna will make it worse. It is also possible for the supply cable to the outbuilding to be damaged

If you are reluctant to do this, try disconnecting all circuits downstream of the RCD except for the sauna. It is not enough to switch off the MCBs because they only break the Phase pole and you may have a Neutral to Earth leak.

So far I think you have been guessing the cause of tripping.

 

[strake]

I am surprised that the company advises replacing the 30mA RCD to a 100mA. This will not give you personal protection.

I had wondered that myself. Why would it not provide adequate protection?

 

[davy_owen_88]

If the 30mA RCD is protecting the sauna, lights and socket outlets then upgrading to a 100mA RCD will not provide adequate protection for the sockets which require 30mA protection.

The problem with most things with heating elements is that there is almost always a small amount of earth leakage anyway. So nuisance tripping is a problem.

Davy

 

[securespark]

100mA will protect property from fire-risk.

However, the level of current above which it is deemed injury or death may occur is 30mA.

 

[strake]

Some very good information...thanks.

Johnd, I wondered about other circuits but they operate fine...I've loaded them all up no problem.
Tripping only occurs after 2 mins of operation of the sauna stove..just enough time to heat up very slightly..

In any event, I take your points on the RCBO/RCD location.

 

[JohnD]

It isn't so easy to guess the state of the other circuits. For example the sockets might have a 10mA leakage, the lights a 10mA leakage, and the sauna a 15mA leakage. Most often earth leakage comes from watery appliances like kettles; from heating elements like cookers and immersions; and from wet fittings like outside lights. Also from rodent damage to cables.