Programmable current activated relay.

[MeldrewsMate]

A friend has no gas to his small flat. It's in Abroadland, so he has electric water heating via a 100 litre tank and a 1.5kW immersion heater.
He wants to install a 6kW electric flow boiler, but his electicity supply limits total power to about 7.5kW. Clearly if the HW is on (1.5kW) and the CH is heating (at max 6kW) too he will be running close to the trip limit. Add a microwave oven in use, or kettle, and nuisance tripping will be common.

I want to measure the input current and switch off the CH when current approaches 30 A (by switching to the 'temperature satisfied' teminal of the boiler. However the problem is that, once the boiler's heaters turn off the load will reduce greatly (by up to 24A), clearly setting up a condition where the 'OK to turn the heating back on' threshold is reached, and an ON-OFF cycle will ensue.
What I need is a relay that operates at the 30A level, but delays resetting itself for a period of time (enough time for the kettle to boil, say), and has variable hysteresis.
The measurement of input current will be via a current transformer giving 1vdc for 50A ac input.

A bonus function would be the ability for it to switch remote sockets off, such as when his gf overloads the system with a 3kW blower heater and hairdryer in the bedroom!

Any ideas?

 

[MeldrewsMate]

I'm going to move this to the electrics outside the UK forum...

 

[ericmark]

I have used current switches to work extractor fans, however I used a PLC to do all the switching, using a PLC you can do nearly anything but not sure if you even need a current switch.

Lived in a caravan for a time, 5A supply, so took supply to water heater element to a relay that switched fan heater off. Plus a switch to turn both off when using kettle or microwave.

So your looking at some where near 25A so first thing is if max is 25A then you want a 20A and 5A MCB or a 15 and 10 amp MCB, once the supply is split into two it is unlikely both will hit max together so your MCB trips before suppliers MCB, next is as with caravan select which item is priority and use relays so when it runs it turns other things off.

I found current operated switches were not that accurate, it may detect some thing taking current over a set amount, but I also used MCB's as set at 5A for fan if fan stalled it could draw 10A, so at 7.5A overload tripped.

A motor overload can show when current is over set limit for a time, rather than just a spike, however remember these are thermal devices and do get hot.

All depends on how much automation you want.

 

[MeldrewsMate]

Thanks for your input.
It's now transpired that he is going to split his incomer into two loads, firstly 6kW to feed the heating, then a fuseboard for everything else. In this way I can directly measure the current drawn by 'everything else', and use an algorithm to determine if the heating is allowed to go on. Later I can modify the system to allow a modulated amount of power to the heating so that the total load does not exceed the trip level.

I won't be using a plc, as there are much cheaper things out there that are much more fun to play with, like the Arduino or Raspberry Pi.

 

[robputt796]

I would personally buy a CurrentCost ENVI, they are a 433mhz CT Clamp thats battery operated (batteries last for years) that you put over your meter tails and they report in the consumption ever 6 seconds, you can then intercept their radio using an Software Defined Radio USB stick like a NooElec and rtl_433 software (all of this will run on a Raspberry Pi), you can then use this to make decisions about switching relays... In my place I have no such need due to the abundance of power (100A supplier feed, 24KW) but I still log my electricity usage in this way so I can shout at the other half when the tumble dryer has been used on a sunny day . Of course 6 seconds is a long time and you'll probably (should almost certainly) trip your supplier MCB way before that.