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DC Circuit Breaker SizingRequirements

Recently there has been a substantial amount of infor-mation released by the Clean Energy Council and otherbodies about the requirements for DC circuit breakerswhen used as the main isolator on a PV array cable.

o understand why the required ratings are differentdepending on the type of inverter installed, and whetheror not the array cables are earthed it is rst necessary tounderstand what happens when a PV array is electricallyconnected to earth on the DC side.

Initially we would point out the following regarding thedifferent types of inverters on the market.AS5033 denes that inverters are either isolated or non-

isolated. Isolated inverters have a transformer (eitherhigh or low frequency) that provides galvanic isolationbetween the AC and DC side of the inverter. Just like ina voltage step transformer there is no electrical connec-tion at all between the two sides and there is no way for acurrent to ow across the gap. Inverters with transform-ers were traditionally the major product available on themarket.

Non-isolated inverters do not have any galvanic isolation,so it is possible that current can ow between the DC and

AC sides. In some cases non-isolated inverters connectthe neutral cable directly to the negative or positive con-ductor of the array.

ransformer-less inverters are generally non-isolated, butsome transformer-less inverters do have a high-frequen-cy isolating transformer so installers should ensure theyare familiar with and understand which inverter productsthey are using.

Connections to Earth

Te basis for sizing the DC breakers in a PV system is toaddress rst-fault conditions. Tis means that when therst unintentional connection to earth occurs, the DCcircuit breaker will be able to safely disconnect the pho-tovoltaic array from the inverter under load conditions.Of course before the rst fault occurs, the device chosenshould be able to safely operate as well.

Floating arrays have an isolated inverter and no inten-tional earth connection on the DC side of the inverter,so under normal operating connections there is no path

for a current to ow other than around the inverter asdesigned. If a single earth fault occurs in such a systemit will be the rst connection to earth, but because thereis only one point of connection there is still no way for

a fault current to ow through the earth. Consequentlyeven under rst fault conditions both poles still share thefull voltage.

When an array is connected to an isolated inverter andis electrically earthed, that is the array has an intentionalrst connection to earth, the normal operating conditionis exactly the same as the rst fault condition from theprevious case. Tese means that one of the array cables isreferenced to earth, but no current can ow through earthbecause the circuit is not complete.

However, when the rst fault occurs in a system that hasan electrical earth it introduces a second connection toearth on the DC side of the inverter. It is then possible forcurrent to ow through the array and the earth fault backto the intentional earth connection so that a completecircuit is formed. Te result of this could be to bypass oneof the poles of the DC circuit breaker so that the entire voltage of the array needs to be broken by a single pole.

Te third option is having a non-isolated inverter with nointentional earth connection. In this system there is noconnection to earth on the DC array cable, but there is aconnection to earth through the inverter. Tis means that

just like the second scenario, when the rst fault occurs,it introduces the second connection to earth into the ar-ray. Tis could again result in one pole of the DC circuitbreaker being bypassed by the earth fault current, or even

An unearthed isolated array be ore (lef) and afer (right) the rst earth ault occurs.

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a DC current being introduced onto the AC side of theinverter. Tis scenario requires either pole to deal with the

full array voltage and current

Remember that having an earthed array with a non-isolated inverter is not allowed according to AS/NZS5033:2005.

Rating DC Circuit BreakersTe different scenarios above demonstrate that the arrayearthing and inverter of a PV system will determine therequired ratings for any isolator used, whether it is a DCcircuit breaker or a double pole DC isolator.

An earthed array with isolated trans ormer be ore (lef) and afer (right) the rst ault occurs.Notice how there are now two connections to earth that could allow a current to ow.

A non-isolated inverter with an unearthed array be ore (lef) and afer (right) the rst aultoccurs. With an earth on the AC side o the inverter a current can ow through the inverter.

Te following table is a summary of what the requiredrating for a breaker is related back to the consequences of

a rst fault forming.

For more information on this topic contact GSES abouttheir DC Circuit Breaker Info Sessions.

Earthing vs BondingTe term earthed array is being used quite loosely in theindustry. Here we are talking about earthing one of theconductors in the DC circuit, NO bonding the PV arrayframe.

Array Conguration Inverter Type Consequence of rst fault Min Rating for EACH pole

Unearthed Array Isolated Inverter No current owing through earth fault- no poles bypassed

0.6 VOC

Earthed Array Isolated Inverter Current will ow through earth fault- one pole bypassed

1.2 VOC

Unearthed Array Non-Isolated Inverter Current will ow through earth fault- one pole bypassed

1.2 VOC

Earthed Array Non-Isolated Inverter Not Permitted by AS5033: 2005