Why Can a Power Adaptor with the Correct Voltage Still Cause a Serious Short Circuit?

When replacing a power adaptor, most people first check the obvious specifications:
▸ Output voltage
▸ Maximum output current
▸ DC plug size
▸ Polarity

For example, if a product originally uses a 24V DC adaptor, it may seem reasonable to assume that another 24V adaptor with sufficient current capability and the same connector should be compatible.

In an audio system, however, that is not always enough.

One critical detail is often overlooked: 
Is the adaptor’s DC output floating, or is one side of the DC output electrically connected to PE (Protective Earth)?

The following example, based on the battery architecture of Broadway, illustrates how an apparently compatible adaptor can create a serious short-circuit condition.

Broadway Battery Architecture

Broadway uses four batteries connected in series:
Battery 1 → Battery 2 → Battery 3 → Battery 4
The battery pack can be simplified as follows:

The connections are:
▸ Battery 1 positive is connected to Broadway V+
▸ Battery 4 negative is connected to Broadway V−
▸ Battery 2 negative and Battery 3 positive are connected together
▸ This midpoint is also Broadway’s Audio Ground

This means Broadway’s Audio Ground is not located at the negative end of the battery pack.
It is the midpoint of the four-cell series battery structure.

This is extremely important.

Normal and Safe Condition

Broadway is charged using the type of adaptor for which the system was designed.
In this case, the adaptor has a:
Floating DC Output

For example:

Neither DC+ nor DC− has a direct low-impedance connection to the building’s PE conductor.

In other words:
Connecting the charger does not force Broadway V− to PE potential.

At the same time, other components in the audio system may use PE as a safety reference or chassis reference without directly shorting their Audio Ground to PE.

Depending on the equipment design, Audio Ground may instead be referenced to PE through:
▸ Resistors
▸ Capacitors
▸ EMI networks
▸ Ground-lift networks
▸ Other impedance networks
Under these conditions, Broadway’s internal battery midpoint remains electrically free to maintain the potential relationship intended by the original circuit design.

The system operates normally.

Dangerous Condition

Now replace the charger with another adaptor.

It may appear to be fully compatible:
Correct output voltage
Sufficient current rating
Correct DC connector
Correct polarity

But it has one critical difference:
DC OUT− is directly connected to PE.

This type of internal connection can be found in certain Class I medical-grade or other special-purpose power adaptors.

When such an adaptor is connected to Broadway:

Adaptor DC-
    │
    └──── PE
          │
     Broadway V-

Because Broadway V− is connected directly to Battery 4 negative:
Battery 4− is now also connected directly to PE.

The Problem Appears Through the Rest of the Audio System

Broadway normally operates as part of a complete audio system.

It may be connected by RCA, XLR, or other audio interconnects to a DAC, preamplifier, source component, or other equipment.

Now assume that another component in the system has its Audio Ground connected to PE either:
directly

or through a:
very low resistance, for example 10Ω

Once that component is connected to Broadway through an audio cable, Broadway’s Audio Ground is also pulled toward PE.

Remember that Broadway’s Audio Ground is the midpoint between:
Battery 2− and Battery 3+

The system now has two different points connected to PE.

The first path is:

The second path is:

PE therefore becomes an external connection between:
Battery 3+
and:
Battery 4−

Battery 3 and Battery 4 are located directly between these two points.

The resulting path becomes:

Battery 3 and Battery 4 are short-circuited through the external grounding path.

This is no longer merely a ground loop or hum-loop problem.

It is a real:
Battery Short-Circuit Loop

Why Can This Be So Serious?

Lithium batteries have very low internal resistance.
During normal operation, current is limited by the intended load, charging circuitry, and system design.

If two series-connected batteries are suddenly bypassed by a low-resistance external path, the short-circuit current may be limited mainly by:
▸ Battery internal resistance
▸ Cable resistance
▸ PCB trace resistance
▸ Connector contact resistance
▸ PE conductor impedance
▸ Audio cable shield resistance
All of these can be very low.
As a result, a very large current can flow almost instantly.

Possible consequences include:
▸ Burned PCB traces
▸ Overheated or damaged connectors
▸ Overheated audio cable shields
▸ Battery protection circuitry activation
▸ Battery damage
▸ Connector damage
▸ Internal component failure
In severe cases, battery overheating may also occur.

This is therefore not an issue of sound quality.

It is a genuine:
Electrical Safety Issue

The Most Common Misunderstanding

A user may reasonably ask:
PE ≠ Signal Ground ≠ Audio 0V ≠ DC Output−

These nodes may be connected together at a particular point in some designs, but they are not inherently the same thing.
▸ Power adaptors
▸ PE
▸ RCA shields
▸ XLR Pin 1
▸ USB Ground
▸ DAC
▸ Preamplifiers
▸ Power amplifiers

A grounding arrangement that appears harmless when one product is examined by itself can create an unexpected current path once the entire system is interconnected through:

You must also verify:

The electrical relationship between the DC output, FG, and PE.