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:

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.
Trev

 

[UNG]

Wired a few tubs with long cable runs and used 10mm swa to overcome the volt drop. I would suspect that a combination of volt drop and load are causing the problems. I have always used "B" breakers on tubs without problem but use a 40A MCB for circuit protection

 

[nickblake]

Alot of hot tubs are wired so they can accomadate three phase hence the three wire and the ones ive seen have had links installed to convert to single phase , dont forget when you size the main RCD ,the RCD is not over load protected so if you use a 63amp RCD then a 32/40 amp MCB your covered , ill never use a main switch so close to the current rating of the Tub as ive seen so many of these with burnt out contacts due to over load