Fuses for Power Inverters

Find out more about fuse protection of power inverters used to convert battery bank DC voltage to mains voltage AC

The Power Inverter (1) is often a key component of all but the smallest renewable energy systems. The inverter takes the DC voltage from a battery (2) or battery bank (3) (typically 12, 24, or 48 VDC) and converts it to an AC voltage similar to that provided by mains electricity (110 VAC in USA, and 220-240 VAC in most of the rest of the world).
A Grid Tie Inverter (4) is different in that it is connected directly to the electricity supply so no batteries are required.

The Importance of Fuses

It is wise to always include a suitable fuse in every connection from the battery bank to an appliance. The fuse is there to protect against short circuits and for overload protection. If at any time the current flowing through the fuse exceeds the rated value of the fuse, it will automatically cut off the power supply. Without a fuse a short circuit can cause a fire or even make batteries explode since the huge currents involved release vast amounts of stored energy.

A power inverter is no different from any other appliance - a fuse should be inserted into the postive cable from the battery bank (as close as possible to the battery bank) to the inverter.

Fuses for Power Inverters

Standard fuses range in values from a milliAmps up to around 35 Amps. These fuses are very cheap since they are mass produced and standard in the automotive industry. (We for example sell in the REUK Shop (5) a 20 Amp Fuse (6) with a rugged holder, connecting leads, and an integrated LED indicator light for under Ł2.00). Inverters however typically draw currents far in excess of 35 Amps and so much larger, less standard, and therefore much more expensive fuses are required.

Pictured above is a typical (250 Amp) power inverter fuse. The fuse itself is the white disk in the center connected to a couple of bolt on terminals. A plastic cover is provided for the fuse holder to keep dirt and dust off, and also to protect against accidental short circuits (for example caused by dropping a metal ruler or screwdriver across the terminals). The package as pictured costs around Ł30 in the UK.

Sizing a Power Inverter Fuse

Inverter fuses are available in a wide range of current values - commonly 110, 150, 175, 200, 225, 250, 300, and 400 Amps, but other values are available.

Selecting the correct value of fuse for any given inverter is not always easy - take a look at the instruction manual for the inverter to see if a DC fuse value is specified. Otherwise you will need to estimate.

For example, a typical 1,000 Watt power inverter can supply up to 1,000 Watts of 240 VAC (in the UK) electricity at around 95% efficiency. Therefore 1,050 Watts of power will be drawn from the battery bank when the inverter is being used to its maximum.
If the battery bank supplies 12 Volts DC, then Ohm's Law (7) tells us that the current is given by dividing the power by the voltage - 1,050 / 12 = 88 Amps.

However most 1,000 Watt rated inverters can supply 2,000 Watts for a short period (10 seconds typically) in order that appliances such as pumps which need a burst of high current to get started can be used. Within 10 seconds a standard 100 Amp fuse would have long since burnt out, and so a 200 Amp fuse would have to be used for such an inverter. BUT, specialist inverter fuses have two ratings - a continuous rating and a blow rating. With this type of fuse, a 100 Amp continuous rated fuse would be perfect.

Buying Suitable Fuses in the UK - Mega Fuses

Obtaining inverter fuses in the UK is quite difficult and they are very expensive. However, there are suitable alternative fuses which are designed for use within large vehicle electric systems with long wire runs where short circuits are a big risk.

Pictured above is a 125A Mega Fuse available in the UK for around Ł5.00. As with all Mega Fuses, the blow rating is double that of the continuous rating making them perfect for use with inverters. This 125 Amp fuse could easily be used with a 1,000 Watt rated inverter connected to a 12 Volt battery bank.

Mega Fuses are bolted onto a suitable Mega Fuse fuse holder such as the 300 Amp rated unit pictured above which costs Ł15-20 in the UK. This can be used with any of the 100, 125, 150, 175, 200, 225, 250, 275, and 300 Amp rated Mega Fuses. An alternative fuse holder is available for use also with the 500 Amp rated Mega Fuse costing under Ł15 in the UK - it is pictured below.

Alternatives to Fuses - Power Breakers

Everytime the fuse fuses - i.e. is broken by excess current - it has to be replaced. If this happens often, it can get very expensive and so a non-disposable / reusable equivalent is required. These are called circuit breakers, power breakers, or trip switches and they are commonly found in home electricity consumer units rated at around 30 Amps. Much larger versions are available for a wide range of uses, and these can be used with power inverters.

Pictured above is a 250 Amp DC Breaker for use with Xantrex power inverters. When a current in excess of 250 Amps passes through the breaker it trips the switch. The circuit can then be switched back on manually after the reason for the current surge has been identified and fixed.

Web Link References

(1) http://www.reuk.co.uk/Power-Inverters.htm
(2) http://www.reuk.co.uk/storage.htm
(3) http://www.reuk.co.uk/Interconnecting-Batteries-for-Battery-Bank.htm
(4) http://www.reuk.co.uk/Grid-Tie-Inverters.htm
(5) http://www.reuk.co.uk/REUK-Products-For-Sale.htm
(6) http://www.reuk.co.uk/buy-20A-FUSE.htm
(7) http://www.reuk.co.uk/Ohms-Law.htm

Article from REUK.co.uk:
http://www.reuk.co.uk/Fuses-for-Power-Inverters.htm
Published: 22nd August 2007
Š REUK 2009