Understanding Cable Assembly Ratings
Pickering Interfaces manufactures a family of cable accessories designed to work with the connectors used on it switching products. The data sheets for these accessories contain information on the cable assembly construction, detailing the materials used and parameters like wire gauge. This in turn defines parameters like capacitance and resistance per meter of cable.
What is less obvious is how this impacts the performance of a cable assembly under load, for example how many wires can carry full rated current at the same time without an excessive temperature rise in the cable. This document explains briefly how we do tests on cable assemblies and explains the specification results we place on the data sheet. The document assumes that the cable assembly is terminated on both ends - has mating connectors fitted - but the same results apply for cable assemblies where one end is not terminated.
How We Test
To test a cable assembly we construct a sample cable, typically 1 meter long, which has embedded in it thermocouples to measures various temperatures. For a cable assembly these might include the following:
- Measurement of the connector shell temperature
This parameter is useful as some users might want to only load the assembly to a point where a user can reasonably expect to handle the connector under load without a burn risk, other users may consider the connector should not be handled during use so higher loads can be applied. We may also measure the temperature inside the connector shell.
- Measurement of the connector pin temperature
This measurement can indicate if the load conditions might create problems in the mating connector (unfortunately manufacturers do not always provide extensive information on this parameter)
- Measurement of the surface temperature of the cable.
The maximum working temperature of the materials used on the outside of the cable assembly might limit how much load current is allowed in the cable assembly.
- Measurement of the cable copper temperature
The copper that carries the current heats up and is insulated from the ambient temperature by other wires and the insulation materials that are used, so it reaches higher temperatures than just a free air wire. The insulation materials immediately in contact with the copper will have a temperature which is the same (almost) as the copper and so this copper temperature might impose a load limit on the cable assembly â€“ much depends on the materials used for the wire insulation.
Having created the sample cable assembly we then perform a set of load tests and record the temperatures. In addition to recording the stabilised temperature we also record how the numbers change with time to gain an estimate of the thermal time constant. Currents flowing in test system vary with time, thermal inertia permits cables to carry higher loads as long as the duration of the load is less than the thermal time constant and the high load condition is followed by a lower load condition so the temperatures can start falling again.
Temperature rise of a connector accessory generally produces two different specification ratings for the maximum load, the higher load is where only the specification of the materials is considered, the lower load rating is where the user has to take into account surface temperatures which might be a burn hazard to user. User temperature limits are defined according to materials in EN61010, in some applications the connector accessory is not accessible to a user (for example behind a closed cabinet door) so it is the materials which establish the limit. The ambient temperature is also a factor, as the ambient temperature rises then the temperature rise permitted to reach the maximum temperature (whether material ratings or burn hazard) becomes lower.
Key Specification Values
This is typically for a single wire and is limited by the connector rating. As more wires are loaded with current temperatures rise and at some stage a maximum load current has to be imposed.
This is limited by the connector used and the construction of the cable (insulation materials, clearance distances)
The data sheet will include information about the recommended maximum working temperature for various parts of the cable assembly. For cables based on the high temperature material PFA then the copper should not get to more than 260C, cables using polyester outer materials should limit to a surface temperature of 100C. How this translates into a load rating however is complicated because of the varying conditions that users might have, including the ambient operating temperature of the cable assembly.
For a cable the power dissipated is primarily dependent on the square of each of the load currents in each wire added together (the power is obtained then by multiplying by the resistance). To avoid having to detail the resistance part of the equation we simply express the rise in temperature for the key parts (typically the connector shell and the surface temperature of the cable) for a unit sum of the square current in the wires.
Thermal Time Constant
The key parts of the cable assembly will include a thermal time constant that can be used to estimate the ability of a cable to withstand a short term higher load than the static conditions would suggest.
What Limits the Maximum Load Specification
There are two primary issues that can impact the maximum current specification.
he first issue is that EN61010 contains specifications on the maximum surface temperature that can be used when a component
is accessible to a user. In the case of a cables assembly this will typically be the connector shell and the cable surface.
The actual temperature limit is not determined by the material capability but rather by the probability of user harm (burns)
created. For that reason metal surfaces have a lower temperature than plastic because they are better conductors and more
likely to transfer heat quickly on contact, thus causing a burn.
However, in a test system the cables may not be accessible to the user, for example if the system is behind a door in a rack. In such cases the ratings can be limited by the materials used and could be higher than those imposed by EN61010. For that reasons some assemblies may allow higher load ratings than would be imposed by concerns about surface temperatures.
In some cases the materials may limit ratings before EN61010 limits are reached, particularly true for lower cost cable assembles where, for example, PVC may be used.
Deriving a Load Condition Maximum
The load will be limited by the first parameter to reach an operating limited so if the user has a concern about the total load current in a cable assembly each limiting factor has to be checked.
Suppose the user has a cable rated at 5A and has 50 wires. Its maximum surface temperature is 100C and the ambient is 25C, so the cable can have a maximum rise of 75C. The data sheet indicates that the temperature rise under load increases by 0.08C per square of current (Amps). Simple arithmetic then suggests the maximum square load current should be 937 Amps. A wire carrying 5A has a square load current of 25, so 37 wires could be carrying 5A and the temperature will rise to a little less than 100C. If the use had an operating temperature which is higher then the maximum load would be more restricted. If all wires were carrying the same current (an unlikely scenario) then the square of the current in each of the 50 wires should be 18.7, or 4.3A.
The user may want to limit operating temperatures for other reasons, and it has to be remembered that if the cables are in a confined area then the ambient temperature will rise. Having forced air past the cables will help keep the ambient temperature under control but is unlikely to have a big impact on the temperature rise per square of current of the cable itself without a significant user effort.
Users may also be concerned about connector shell temperatures, as this rises then a user handling the shell under high loads may risk a skin burn, so the same methodology should be applied to the connector shell if this is a risk. If for example the user wants the shell restricted to a 35C rise and the shell has a 0.044C rise per square of current then in the above example the sum of the squares of the current needs to be limited to 795A.
Short Term Overloads
The thermal inertia of the cable assembly means that a cable can be more highly loaded for short time intervals provided the average remains within limits and the time that this occurs is less than the thermal time constant of the critical parts of the assembly. If for example the thermal time constant is 5 minutes the loading a 50 wire 5A assembly for one minute followed by no load for one minute is likely to be a safe operating condition for many applications as the sum of the squares is 1250A, the duty cycle is 50% so the average is 625A and the time constant is significantly less than the thermal time constant for the key parts.