Hot tub trips MCB

[Trev2020]

Hi guys, I’m after some advice. I have just run a supply for a hot tub for a family friend as follows:

Untitled

• Trev2020
• 7 Nov 2010
• 1

Everything tested ok and the hot tub powered up fine when I was there although I didn’t run it for any period as there was no water in there. The following day the owner was running it and it was fine for 30 mins then the 20A MCB tripped.

They rang me and I told them to switch everything off and turn on the MCBs/RCD as follows
1. 20A MCB
2. RCD
3. 16A MCB

Now every time they switch on the 16A MCB the 20A MCB trips. If there was a fault with the hot tub surely the 16A MCB or RCD would trip first and not the 20A MCB? I’ve used type B MCBs, would the pump starting current cause these to trip?

I’m newly qualified and would just like to get some advice as to why this is happening before I go and have a look. Any advice would be greatly appreciated.

 

[Adam_151]

a B16 breaker will not dicriminate with a B20*, though normally that would manifest itself in both breakers dropping out, rather than just one, though if the B20 is a particualy keen one from the batch and the B16 is somewhat sluggish, who knows!

A delay of 30mins would tend to suggest an overloading issue, what is the rating of the hot tub? Though as you say it now trips immediatly after reseting the 16A breaker? then there could be thermal damage occured to a terminal at some point which has now gone S/C


*You struggle to get MCBs to discriminate at the best of times, Its not unhead of for a fault on a circuit protected by a 20A B type, to take out the 63A C type supplying the submain

 

[electronicsuk]

Did you ask what loads are connected in the shed? I assume the DB in there feeds additional sockets and the like, so you're cutting it fine with a 16A load when the submain to the shed is only rated to 20A.

 

[Trev2020]

Adam, there was no rating on the hot tub instructions, though the installation instructions option were either;

1 x 32A feed into line 1 with internal link to line 2,
2 x 16A feed into each line,
1 x 16A RCD protected feed into line 1 with internal link to line 2.

Theres a heater rated at 2kw and a pump and LED light.

electronicsuk, there are no other loads connected to the consumer unit as installed it as part of the installation.

If you suspect thermal damage would the best plan be to disconnect the feed to the tub and then do continuity / IR tests along the cables / shed consumer?

If I remember correctly there were separate plugs for the equipment coming of the control box, if I find no faults in what i've done would unpluging all these and then slowly reconnecting them one by one till it trips be a good idea, at least that way I can see if the pump / heater as gone faulty?

Thanks for the replies.

 

[ban-all-sheds]

1 x 32A feed into line 1 with internal link to line 2,
2 x 16A feed into each line,
1 x 16A RCD protected feed into line 1 with internal link to line 2.

The last is inconsistent with the first two....

 

[Trev2020]

Sorry, forgot to mention that if the last option was used then a jumper needed to be moved on the PCB in the control box.

As there was only 1 option to install with a RCD, then I went for that. The hot tub is Canadian so I thought the first two options were for countries where perhaps RCD protection wasn't required?

 

[Adam_151]

There is nothing to stop you RCDing however it is wired, why the documentation is set out like that I do not know

We really need to know some more information on the device, AFAIK canada; like most of the states uses a 110v split phase system, with 220v present between the phases. and it rotates at 60hz instead of 50hz. So we need need to know a bit more about to speculate about suitabilty and wiring on a british supply (of course, it could be designed for the export market, but we do not know at this stage)

 

[Trev2020]

It is a second hand unit and it was previously connected with via 6mm2 T&E into line 1.

Wish I had just ran in the 32A supply

I've asked the owner to send my a picture of the circuit diagram on the lid of the control box so I can have another look at whats connect to the control panel.

Thanks for all the advice