Modifying SCT013 to remove Burden Resistors

Guys,

If anyone else ends up in the same boat as me with a heap of SCT013 15/30/50 that return a voltage instead of a milliamp reading - then it is a ridiculously easy fix to remove both (the 15amp units only seem to have one) burden resistors and takes no more than 2 to 3 minutes per unit.

Make sure you have a soldering iron with a nice sharp tip and a pair of helpng hands makes everythign go much more easily.

  1. From the bottom push the cable up into the housing a couple of centimeters.
  2. Open the clamp - looking from the top you will see two small blue tabs - this is all that holds the PCB and split magnet unit in the housing
  3. I have found the easiest way is to slide a small jewellers screw driver down between the housing and the PCB and just slightly angle the PCB forward and away from the tabs
  4. If you have pushed enough cable through from the bottom this will provide enough upward force to move the PCB out from under the restraining tabs
  5. SLowly feed more cable in and the unit will come loose from the housing.
  6. on the PCB you can see the two resistors (they are tiny surface mount units)
  7. Place the PCB in your helping hands clamp and hold the sharp point of your heated soldering iron agains one side of the solder on the SMD resistor and apply a gentle force with the tip of the soldering iron pushing the resistor towards the other side - as the solder melts the resistor will come free of the PCB and bend upwards - you can then push it bakwards and the legs on the remaining side will snap free.
  8. once you have both of these resistors free thats it - slide the unit back together until the PCB is resting under the two restraining tabs again.

Here is a reference for this - this shows a picture of one with the two resistors removed

These appear to track exactly the same as the SCT013-00 (100 amp) unit and to very closely align with an external Killawatt meter

Craig

There is a tiny difference in that for some reason the voltage type current ratio is 1860:1 rather than the 2000:1 of the SCT013-000. So if you configure them (with the burden removed) as an SCT013-000, they will read about 7% higher. Best to configure as generic with 1860 turns and phase about 2.5.

Excellent thanks - will try and advise

Craig

If the burden resistors are removed won’t they be vulnerable to infinite voltage spikes if they are unplugged while clamped around a live wire?

While it’s always good practice to never leave a CT unplugged for any length of time, those protected with a TVS diode can be safely left open for short periods while installing and/or reconfiguring.

These CTs have TVS diodes in addition to the internal burden resistor(s). That’s pretty much belt and suspenders, and @craigcurtin has left the TVS diode in place. It is the black component above where the burden resistor(s) were. These are the same TVS diodes that YHDC uses in their current type SCT013-000.

Bob,

Just an update on this - got time to get back to it today.

I have modified about 3/4 of my clamps to remove the Burden resistors.

I took a sacrifical extension lead, stripped off the outer sheath to expose the active wire.

Placed 3 diferent CTs on there - all of them the Voltage type from YHDC that i had removed the burden resistor from

I plugged this extension lead into the output of a Watt Meter and a hair dryer into the other end

I initially tried with a 15a, 20a and 30a CT - all of them on the same wire together and into discrete inputs on the IOTAWATT

I set each of them back to having the standard 20ohm burden resistor in the setup screen

Interestingly each of them required a different number of turns specified to return the true voltage that lined up with what the Power Meter was seeing

15a = 1820 phase lead 2.5
20a = 2040 phase lead 2.5
30a = 2390 phase lead 2.5

I tried this on a number of them in each Amp range and once dialed in with the above settings they were all very close to giving the same answers (within 10w or so)

I still have some 50amp ones to do

Let me know if you want any more testing done for your records as i am sure some other idiot (like me) will end up purchasing these as well

Craig