Why is my inverter drawing such a high current on first power up?
MECO Technical Note MTN 0030
Topic: New inverters creating high inrush currents on initial connection to the DC power source.
Similar Questions:
This MECO Technical Note serves the following frequently asked support topics.
- New inverter blows its fuse?
- New inverter drawing high current without loads?
- Inverter will not power up, just trips the breaker?
- Why is my inverter drawing such a high current on first power up?
- New inverter seems to be drawing a high current even without any load, is my inverter faulty?
- I checked all my wiring but when I installed the fuse for the first time there was a huge arc and the fuse has now blown. The inverter is not even switched on, there are no AC loads.
Answer
An inverter DC input can look like a near short circuit for the first instant because its capacitors are discharged. See below for the solution.
Why it matters
Any un-controlled first connection of a new inverter causes a huge inrush current, this can cause a arcing (a large electrical spark) this can in turn damage contacts, stress a BMS, trip overcurrent protection such as fuses or breakers, damage batteries, cause injury, damage eyesight, cause burns, fire, or explosion.
Technical explanation
Inverters use several large capacitors on their Direct Current input (from the batteries). When the inverters are new these capacitors are effectively drained - so connecting a new inverter for the first time can look like a short circuit to the electrical system.
In order to prevent tripping the Over current protection, or damaging the contacts of a switch, or battery terminal, simply control the rate of flow - the current when connecting an inverter for the first time. We can do this by 'soft starting' the inverter with a pre-charge resistor — or in some cases an incandescent bulb — limiting the current while the capacitors charge.
Some manufacturers provide a pre charging lead for this purpose.
Definitions
Capacitor: An electrical component that can very quickly store and release electricity, used in a variety of electronic devices especially inverters, but also solar controllers, and dc to dc controllers. Capacitors are similar to batteries in some respects but when discharged they have virtually no resistance so they can draw incredibly high currents when empty.
Inverter: A device that converts lower voltage DC Direct Current usually from batteries to higher voltage Alternating Current (like the mains power in your house).
Common mistakes
General
Any uncontrolled first power up of a new inverter without first pre charging the capacitors.
Installers should be very aware of how this event of first power up takes place, and what methods are in place to ensure that no damage is done to the components that make this first connection, or the vessel as a whole. As mentioned - it is safer even if not required by the manufacturer, to pre charge the inverters capacitors first. It is also part of the MECO Standard to do so in all cases with inverters and solar controllers.
Common mistakes
Specifically
Some examples of common mistakes and the potential damage caused.
Dabbing the terminals
A common mistake is to quickly dab the battery terminal, if a high current flows this can weld the terminals together, or it can generate very high temperatures in a very small surface area - this can vaporise some of either or both of the contact materials and cause fire, explosion or injury. Also if the batteries are Lead Acid and in poor condition - this high loading may be their last event - the sudden inrush could cause the battery to pop - and you dont want to be near the battery if this is a risk.
Creating any ignition sources in a sealed poorly ventilated space, near lead acid batteries is never a good idea. If batteries are making an eggy smell - this gas is heavier than air - toxic and explosive.
Using the battery switch to make the first power up
If this is in good condition and rated high enough, then it may be okay for a first power up but if it is of poor design, or ageing the sudden inrush may weld the contacts together.
Installing the fuse to make the first power up
Installing the fuse to make the first connection without pre charging the capacitors could damage the fuse holder or the fuse. In any case it will cause an arc and this can lead to injury, and eye damage.
Standards note
ISO, ABYC, MECO
Technician takeaway
Wear appropriate PPE for the first connection of any DC device with large capacitors, such as full face mask, eye protection, long gloves.
Ensure that any areas you are working in are well ventilated, with no unusual smells, and consider the use of a hand held gas alarm if the vessel is not equipped with a gas alarm.
Expect there to be an arc, even with pre charging.
Never connect any DC device with large capacitors without first pre-charging.
Pre charge any DC device that has large capacitors via a low wattage incandescent lamp rated at the correct voltage, or a dedicated pre charging lead with built in resistance.
Pre-charge prior to fitting the inverter fuse, across the fuse holder, or across an open breaker if a breaker is used.
Covered in MECO training
MET 1 & 2 Package is recommended if you are doing work on small inverter systems however if you are working with lithium batteries, or any other battery system that requires electronics to function safely then you are required to complete the MET 3 unlimited course - now know as MET 4