Current draw indicator.

[The Red Bladder]

Does anyone make a simple device that tells me if a device 'downstream' of a switch is actively drawing power?

This is for a new pressurised immersion heater that is built into a wall and is not easily accessible. The switch is in our utility room, along with an RCD, etc., etc. It would be useful to know if the thing has heated up the water and switched itself off, without all the faff of installing a Wattmeter and explaining how others are to use the thing.

It seems a simple thing to have but I have not been able to find anything that performs this simple task.

[ef37a]

I made something similar to check the duty cycle of Weller Magnastat solder pencils. I used the 'current transformer' principle. I had a ~25mm ferrite toroid with perhaps 100 turns on it and wound on about 10 turns of thicker wires. I used a resistor in series with a LED on the 'secondary' to indicate current flow.

You could surely cobble something similar but of course it would have to be safely enclosed.

I found that a good iron gave a near perfect 50/50 on off duty cycle and was very regular. An iron that was well outside that figure and random needed a new switch. I looked after about 50 stations.

Dave.

[MarkOne]

Something like this perhaps?

https://amzn.eu/d/0JM5jSl

[ef37a]

Something like this perhaps?

https://amzn.eu/d/0JM5jSl

I was under the impression that Red's heater was hard wired not on a 13A outlet?

Dave.

[MarkOne]

Hmmm. Yes, I see.

I have a current clamp meter that would do the job, but you do need to separate the live and neutral conductors and put the clamp on the live wire only.

[Wonks]

You cold put a temperature sensor on the cable. It's quite a high current with a 3kW immersion, and on our old hot water tank, the immersion lead got noticeably warmer when on.

But you then need a sensor with a logging function.

[Arpangel]

One of these maybe?

https://www.screwfix.com/p/lap-ac-dc-d ... lsrc=aw.ds

[Wonks]

One of these maybe?

https://www.screwfix.com/p/lap-ac-dc-d ... lsrc=aw.ds

As MarkOne said, to use one of those you need to separate the wires out and put it round either the live or neutral. With a two/three core cable, the current goes in both directions along the L and N, which cancels the meter's signal out. Clamp round either the L and N and you get a reading, but clamp round both and you don't.

Of course Bladder could sacrifice the existing cable to pull the wires apart, and once he knows it either is or isn't switching on and off, then replace it with a new one.

[James Perrett]

You cold put a temperature sensor on the cable. It's quite a high current with a 3kW immersion, and on our old hot water tank, the immersion lead got noticeably warmer when on.

But you then need a sensor with a logging function.

There seem to be lots of temperature loggers which are mainly aimed at the food industry. However, wouldn't it be better to remotely display the water temperature in the tank (or even the air temperature in the tank cupboard)? You could use a weather sensor in the tank cupboard or something like this

https://uk.rs-online.com/web/p/data-loggers/1993854

in conjunction with a phone or tablet.

[Wonks]

That particular RS sensor is OK for monitoring out of limit temperatures (which I suppose you would get if the thermostat isn't cutting off the immersion heater at the right temperature), but otherwise than sensing out email alarms it only seem to give you max min and average temperatures, rather than a logged value over time.

A lot of the simpler data loggers have fixed time intervals, say 30 minutes, which is too long a period IMO for that sort of analysis.

If I wanted to know if it was switching, I'd just take the cover off the immersion heater (assuming it was accessible), adjust the thermostat so that it switches off, check there's a near-mains voltage across the thermostat connections, then adjust the thermostat so that it switches on and check there's no voltage differential across the thermostat connections. You then know the thermostat is working and switching power. What you don't really know is how well calibrated the thermostat setting is, and 60°C could actually be 50°, 55°, 60°, 65° or 70°C etc.

[ef37a]

One of these maybe?

https://www.screwfix.com/p/lap-ac-dc-d ... lsrc=aw.ds

As MarkOne said, to use one of those you need to separate the wires out and put it round either the live or neutral. With a two/three core cable, the current goes in both directions along the L and N, which cancels the meter's signal out. Clamp round either the L and N and you get a reading, but clamp round both and you don't.

Of course Bladder could sacrifice the existing cable to pull the wires apart, and once he knows it either is or isn't switching on and off, then replace it with a new one.

Quite, and my current transformer idea still requires access to one conductor.
I have found a couple on the RS Comps' site but you would still need to build the LED secondary circuit.

Dave.

[Folderol]

The cheapest way to do this is with a bridge rectifier, a couple of diodes, an LED and resistor, but again it means breaking into the circuit. Depending on exactly where and how the switch is mounted, this would be the best place.

[Wonks]

You could even just wire a 230v lightbulb across the thermostat if the terminals could cope with two wires.

[ef37a]

You could even just wire a 230v lightbulb across the thermostat if the terminals could cope with two wires.

Ah now! Most immersion heater switches had a neon indicator, certainly the ceiling pull switch in my bathroom did. Might be possible to route the neon's wire to the output side instead of just indicating mains present?

Or you could use one of the boxes used for storage radiators wired in downstream. They have a neon in them as a rule.

Dave.

[Arpangel]

How inaccessible exactly is this immersion? If the thermostat has gone then you’re going to have to get at it anyway.
Also, these do make a faint noise when they are working, if you can get a long stick to touch the end of it, put your ear to the stick and you’ll hear a faint whooshing and bubbling sound, it’s very obvious that it’s on.
Also, I’m not joking, but if you are able to stick a cheap contact mic to the tank, you’ll definitely hear it.

[Wonks]

You could even just wire a 230v lightbulb across the thermostat if the terminals could cope with two wires.

Ah now! Most immersion heater switches had a neon indicator, certainly the ceiling pull switch in my bathroom did. Might be possible to route the neon's wire to the output side instead of just indicating mains present?

Or you could use one of the boxes used for storage radiators wired in downstream. They have a neon in them as a rule.

Dave.

The neon is between live and neutral on the switched side of live (so it is on the output side). You can’t move it so that it shows if the thermostat is making and breaking and putting the heater on load. That can only be done at the thermostat end of the circuit.

[ef37a]

Doh! Yes Wonks of course. Bugger.

Dave.

[Folderol]

If the thermosat is in an accessible/visible location, you could put the neon across it's contacts. It will come on when the thermostat is off.
Don't forget that if its a 'bare' neon, you need to put a resistor (about 1M) in series with it.

[The Red Bladder]

The idea was to have a neon (or LED) that lights up when the current is flowing and goes out when the thermostat has reached full temperature. The wires are behind plaster and it would not be practical to start digging it apart.

The switch and its RCD are in a utility room downstairs and the immersion heater is buried in the wall of an upstairs bedroom behind a bulkhead. The current draw is 3kW or 6kW, depending on whether one or two elements are wired up. At the moment, just one is wired up as the switch is just a 20amp thing. It will be replaced with a 50amp switch next week.

[Wonks]

Then you'll need to work at the RCD end. Somehow you'll need to fit a current transformer around the live cable and attach that to a current sensing relay, and use the relay contact to switch a light on and off. I can't think of an easier way to do it.

[Arpangel]

At some point you’re going to have to gain access, immersions don’t last long. Maybe worth finding a new location for it.

[ef37a]

Then you'll need to work at the RCD end. Somehow you'll need to fit a current transformer around the live cable and attach that to a current sensing relay, and use the relay contact to switch a light on and off. I can't think of an easier way to do it.

Yes. You should find a current transformer will produce enough output volts to light a red LED. You need a diode in series with the LED and in theory a limiting resistor but the DC resistance of the transformer should suffice, bit of calculation for you!

I dare say you will need two or three turns through the transformer primary side to give about 3V output. My iron lashup worked for just a 2A current.

Dave.

[Wonks]

I dare say you will need two or three turns through the transformer primary side to give about 3V output. My iron lashup worked for just a 2A current.

Though I doubt there's enough spare cable to wrap around even once, let alone three times. You'd probably have to extend the wires to do so. Not something I'd recommend doing with chocolate block with immersion heater currents. You might be able to fit a couple of DIN rail terminals in the consumer unit for a professional install.

[Folderol]

I dare say you will need two or three turns through the transformer primary side to give about 3V output. My iron lashup worked for just a 2A current.

Though I doubt there's enough spare cable to wrap around even once, let alone three times. You'd probably have to extend the wires to do so. Not something I'd recommend doing with chocolate block with immersion heater currents. You might be able to fit a couple of DIN rail terminals in the consumer unit for a professional install.

At 50A a straight through wire should be sufficient. You can also get bidirectional LEDS (two in reverse parallel so no need for the series diode - still advisable to have a series resistor.

Transformer
https://uk.rs-online.com/web/p/current- ... rs/1243897

AC LED
https://uk.rs-online.com/web/p/leds/2285821

[Wonks]

2 x Single phase 3kW elements, so around 12.5A each. Though I’d have thought with two elements in the hot water tank they’d be installed at different heights and operate independently. That’s been the case on the HWS cylinders I’ve worked on in the past, though they were almost certainly bigger cylinders for bigger buildings. So depending on the arrangement there’s no guarantee that you’d get 25A, except at times of greatest water draw off when the tank temperature dips.

You could have two CTs of differing values to drive two independent LED circuits, one for one element on the other indicating both, or you could could probably use a single CT and two current sensing relays to do a similar job.

A lot depends on the space in the distribution board. A 50A+ capable wire (as it will need to be rated higher than its breaker) isn’t going to be very flexible, and given a minimum bend radius for the cable, a straight-through arrangement for the CT is probably going to be the only one that’s viable. Probably 10mm2, maybe 16mm2 depending on how long the path and how enclosed it is in its route to the immersion heater.