Discrimination problems

[DIYer]

Hi,
I have a 3 phase supply with each phase going to it's own consumer unit. There is a 3 phase isolator between the supply and each board. The isolator has 32a BS88 fuses on each supply. My problem is that the fuses in the isolator are blowing before the RCBO's are tripping on the single phase boards. I had one phase go down, so I decided to change all the fuses in the isolator as the original fuses were very old. This time a different phase has blown. My problem is the load on that phase is lower than that of the fuse in the isolator, should I have higher rated fuses in the isolator before each board?
I have some basic electrical understanding but thought it would be wiser to ask before replacing the fuses, any help would be very helpful
Thanks

 

[mrloy99]

Hi,
I have a 3 phase supply with each phase going to it's own consumer unit. There is a 3 phase isolator between the supply and each board. The isolator has 32a BS88 fuses on each supply. My problem is that the fuses in the isolator are blowing before the RCBO's are tripping on the single phase boards. I had one phase go down, so I decided to change all the fuses in the isolator as the original fuses were very old. This time a different phase has blown. My problem is the load on that phase is lower than that of the fuse in the isolator, should I have higher rated fuses in the isolator before each board?
I have some basic electrical understanding but thought it would be wiser to ask before replacing the fuses, any help would be very helpful
Thanks

What is the loading on your consumer units?Have you checked actual loading with tong tester?Perhaps you have a faulty rcbo which should be tripping and isn't--maybe you have an intermittent fault on cable or equipment which only shows after it heats up.ie,maybe you have a short circuit fault rather than an overload which you suspect at the moment.

 

[DIYer]

There is 26a on the consumer unit consisting of one 20a rcbo which has two air con units and a 6a rcbo with an extractor fan. I have not checked actual loadings with a tester yet and all equipment is brand new. All three consumer units have been up and running around six months with no problems other than fuses in the isolator blowing.
Thanks for the reply.

 

[mrloy99]

There is 26a on the consumer unit consisting of one 20a rcbo which has two air con units and a 6a rcbo with an extractor fan. I have not checked actual loadings with a tester yet and all equipment is brand new. All three consumer units have been up and running around six months with no problems other than fuses in the isolator blowing.
Thanks for the reply.

Perhaps each of your air con units take more than 10amp.Is it the split type with an indoor and an outdoor unit (both single phase) or are they 3phase.I think you would need to check actual loadings under a variety of conditions,heating and cooling.You might find that they each take 16amps and problems only occur when they both kick in together,Though as you point out the 20amp should be operating first

 

[DIYer]

Hi, thanks for the reply again.
I have had the installation tested today and everything seems fine. I was told it could be faulty fuses in the isolator, what are the chances of this? Fingers crossed I should have no more problems.