Another pair of San Luis Obispo County union groups, representing 135 workers in law enforcement, have agreed to a two-tier pension plan for new hires, inching county management close to 100 percent agreement with its unions on the two-tier system.

After the move Tuesday, bargaining units representing 96 percent of the county workforce have now signed off on the two-tier approach.

Under the two-tier system, new employees generally receive less generous benefits than their predecessors doing the same job.

On Tuesday, the Board of Supervisors approved the proposal with the Association of San Luis Obispo County Deputy Sheriffs. It will apply to employees hired in that union’s two units after Sunday.

The only unit not to adopt the plan is the one that represents probation peace officers, which includes juvenile services officers, deputy probation officers and supervising deputy probation officers, according to Human Resources Director Tami Douglas-Schatz.

Deputy district attorneys and deputy county counsels also fought the bifurcated system, but the Board of Supervisors imposed the second tier on both groups in August 2011 after bargaining ended in impasse.

There are 176 employees already covered by the second tier.

The Association of San Luis Obispo County Deputy Sheriffs includes deputy sheriffs and senior deputies. It is not to be confused with another law enforcement bargaining unit, the 146-member Deputy Sheriffs Association, which includes correctional deputies, a correctional sergeant, dispatchers and such others as a forensic specialist and property officer.

Some of the changes in the two-tier system enumerated by Douglas-Schatz:

• Under tier one, the final retirement compensation is based on the single highest year salary; under tier two, it is based upon the highest three-year average.

• Under both systems, retirement earnings may reach 90 percent of income.

• Under tier one, there is a 3 percent retiree cost-of-living adjustment; tier two limits the retiree cost-of-living adjustment to 2 percent.

• Tier one retains the deferred retirement option plan (DROP); tier two eliminates it. DROP is an optional, voluntary program that allows employees to have retirement benefits deposited in a special investment account and cease making contributions to the Pension Trust, while they continue to work in their current positions.

It is a voluntary method of receiving a distribution of retirement benefits; it is not an additional retirement benefit.

• Under both plans, there is a 50-50 cost sharing of any pension rate increases as adopted by the Pension Trust Board after receipt of actuarial data.